Effect of defatted oilseed meals applied as organic fertilizers on
vegetable crop production and environmental impact
Marco Mazzoncini(1)*, Daniele Antichi(2), Silvia Tavarini(2), Nicola Silvestri(2), Luca Lazzeri(3),Lorenzo D’Avino(3) (1) Centre for Agro-Environmental Research “Enrico Avanzi”, Via Vecchia di Marina 6, 56122 San Piero a
Grado (PI), Italy
(2) Department of Agriculture, Food and Environment – University of Pisa, Via del Borghetto 80, 56124 Pisa,
Italy
(3) Council for Agricultural Research and Economics, Research Centre for Industrial Crops CRA-CIN, Via di
Corticella 133, 40129 Bologna, Italy
*corresponding author, e-mail address: marco.mazzoncini@unipi.it, telephone number: 00390502210504, fax number: 00390502210513
Abstract
Defatted oilseed meals from sunflower (Helianthus annuus L.), and some Brassica species have a well known economic value as feed for animals, whilst their value as organic nitrogen fertilizers has been not fully explored so far. Compared to sunflower, the seed meals of Brassica species are reported to have a potentially higher capacity of nutrient supply, due to their lower content of indigestible fiber. The high content of glucosinolates may conversely reduce the availability of nitrogen, because of their inhibitory effect on nitrification processes.
In this lysimetric study we compared the effect of the application of three seed meals: partially defatted meals of sunflower and of Ethiopian mustard (Brassica carinata A. Braun), and completed defatted meal of sunflower on the yield and N utilization of three vegetable crops (lettuce, chard and spinach), compared to an unfertilized control and one fertilized with ammonium nitrate. Furthermore, we also investigated the residual effect of fertility of oilseed meal application on winter barley and fallow. Both sunflower meals revealed a fertilization effect comparable to that of the mineral fertilizer, whilst B. carinata meal reduced the yields of chard and spinach, due to low N availability as shown by the lower N use efficiency index. We 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
argue that this lower efficacy of B. carinata meal was due to the technique of the application of the meal, that might have produced a high concentration of glucosinolates in the soil layer explored by roots, concurrently determining a significant impact on soil biota and direct phytotoxicity phenomena. A relevant part of the N applied with fertilizers was not absorbed by the three vegetables and hence accumulated in the soil. Despite the high values of unabsorbed soil N in the plots fertilized with oilseed meals, nitrate leaching was lower than with ammonium nitrate. The impact on global warming potential of fertilization with defatted oilseed meals was assessed and shown to be lower than or comparable to the impact of the synthetic fertilizer. In particular, defatted sunflower meals caused a lower impact per hectare and total biomass, and a similar impact per fresh marketable product.
Overall, our findings confirmed the high value of oilseed meals as a sustainable alternative to mineral fertilizers. However, their efficacy strongly depends on the technique of application to the soil.
Key words: oilseed meals, nitrogen fertilizers, lettuce, chard, spinach, GHG emissions
1. Introduction
The external input reduction, the use of renewable sources and the improvement of the naturally occurring processes within the agricultural systems are probably the main key actions involved in reducing the environmental impact of agriculture, making it more sustainable. In this context, the use of oilseed meals as nitrogen (N) organic fertilizers may represent an opportunity to reduce the environmental impact related to chemical fertilizer production and use (high fossil energy use, massive greenhouse gasses emission, N leaching, nitrous oxide volatilization, etc.), and to improve soil fertility through organic carbon (C) supply and soil biological activity strengthening (Zaccardelli et al., 2013b). Even if oilseed meals are generally used in animal feeding, several studies have documented the efficacy as fertilizers of seed meals from sunflower (Helianthus annuus L.) and some Brassicaceae, e.g. Ethiopian mustard (Brassica carinata A. Braun) (Gale et al., 2006; Marchetti et al., 2008). Brassicaceae oilseed meals represent a particular type of meals; despite the similar N and phosphorous (P) content with respect to sunflower, no genetically improved seeds contain 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
glucosinolates (GLSs) that through enzymatic hydrolysis via myrosinase produce a series of biologically active compounds (mainly isothiocyanates) (Fahey et al. 2001) that have shown an effect in controlling weeds (Ascard and Jonasson, 1991; Vaughn and Berhow, 1998; Vaughn et al., 2006; Rice et al., 2007), insect pests (Elberson et al., 1996, 1997), nematodes (Walker, 1996; Lazzeri et al., 2009), and soil-born pathogens (Smolinska et al., 1997; Manici et al., 1997; Mazzola et al., 2001; Chung et al., 2002). On the other hand, some authors demonstrated also significant allelopathic effects on seedlings of crops seeded after soil incorporation of brassicaceous crops with high GLS content (Jafariehyazdi and Javidfar, 2011), whilst for transplant crops, such as field vegetables, only minor effects on plant growth are reported (Haramoto and Gallandt, 2005; Ackroyd and Ngouajio, 2011). In any case the biocidal activity was clearly related to GLSs-type and concentration (Brown and Morra, 1997; Vaughn, 1999; Bending and Lincoln, 2000). Some
glucosinolate-degradation products such as 2-propenyl isothiocyanate (allyl isothiocyanate, AITC) and ionic thiocyanate (SCN-) showed a general toxicity to microorganisms (Beekhuis, 1975; Brown and Morra, 1997; Vasquez et al., 2006), including those responsible for the nitrification process (Bending and Lincoln, 1999; Vasquez et al., 2006; Kim et al., 2007; Brown and Morra, 2009; Snyder et al., 2009, 2012; Reardon et al., 2013). Omirou et al. (2011) stated this detrimental effect on nitrification processes was more the results of a change in soil biota community due to the incorporation of fresh biomass rather than of direct toxicity of GLSs on microorganisms. Furthermore, Goos et al. (2009) reported for meal from crambe (Crambe
abyssinica L.), a brassicaceous species similar to B. carinata but with a different composition of
glucosinolates, only short-lived inhibition of nitrifying bacteria, drawing the attention to the chemical composition of oilseed meal and to the kinetics of its mineralization as explanatory variables of reduced presence of nitrates in the soil. Likewise Hollister et al. (2013) demonstrated that also the long-term suppression of soil borne pathogens produced by the application to the soil of brassicaceous oilseed meals was due to a shift in microbial community structure and composition.
Many of these reported studies regarded the effect on crop productivity of defatted oilseed meals (DOMs) originated by chemical oil extraction with hexane, and used as amendments or fertilizers, but not so many specific studies have been carried out on a particular kind of DOMs, i.e. defatted oilseed cakes. Even if the 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77
term cake generally indicates “the solid residues after oil extraction from seeds”, following the early spreading of mechanical oil extraction systems suitable for farm or cooperative plants, it has now assumed the meaning of “the solid residues obtained after pressure oil extraction from seeds”. The term “cake” implies thus a different production system based on the use of non-chemical agents for oil extraction, locally managed and overall characterized by lower environmental impact. In terms of quality, due to lower aggressiveness of mechanical extraction compared to hexane, cakes are only partially defatted, containing up to 8-12% of residual oil that may affect also nutrient availability, especially N and P. On the one hand, the presence of oil may decrease crop nutrient availability through the slowdown of mineralization processes. This may occur because of the hydrophobic behavior of the lipid fraction, which prevent the imbibition of the material after soil incorporation, or because of the inhibition of soil microbial activity due to the
presence in the oil fraction of toxic compounds, such as polyphenols (Zaccardelli et al., 2013a). On the other hand, the slow mineralization rate caused by the presence of residual oil may lead to a slow release of nutrients from the organic pool into the plant-available forms, thereby reducing nutrient losses (e.g. through nitrate leaching or soil run-off) and better meeting the needs of the crops along their growth cycle (Moore et al., 2010). Furthermore, the presence of residual oil increases the C:N ratio of cakes compared to completely defatted oilseed meals, favoring immobilization processes and soil organic matter increase. Nevertheless, the use of organic fertilizers is strongly encouraged especially in Mediterranean conditions to counter the soil organic matter depletion by mineralization, also reducing N2O emissions (Aguilera et al. 2013). In addition, valorization of biodiesel by-products could increase the economic and environmental sustainability of the entire biodiesel chain, reducing its impact on global warming potential (GWP). Agriculture contributes 10-12 % of greenhouse gas (GHG) emission and GWP is a priority in agro-food consumption (Page et al., 2012); GWP is also the impact category in Life Cycle Assessment (LCA) methodology most widely affected by N fertilization rate in lettuce (Romero-Gamez et al., 2014). In this lysimeter study under Mediterranean climatic conditions, we compared three different oilseed meals, completely defatted (as obtained by chemical oil extraction process using hexane as solvent) and partially defatted (as obtained by mechanical oil extraction process using 35-40 MPa pressure), applied as N 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103
organic fertilizers to leafy green vegetables grown in sequence (Lactuca sativa L., Beta vulgaris L. and
Spinacia oleracea L.). Assessments included effects on crop yields, nitrogen use and environmental impact.
Environmental impact assessment was carried out comparing a cropping system managed by applying conventional fertilizers with three others managed entirely by applying meals and cakes obtained from the biodiesel chain. Environmental assessments included: (i) carbon footprint, (ii) residual fertilization effect on subsequent barley and (iii) nitrogen leaching.
2. Materials and methods
2.1. Study site and experimental design
Beside the above-mentioned organic fertilizers (completely defatted sunflower meal-CDSM, partially defatted sunflower meal-PDSM and partially defatted B. carinata meal-PDBM), the experiment included other two treatments: the unfertilized treatment (control) and the “conventional” treatment based on mineral fertilization (AN=Ammonium Nitrate). The five treatments were randomly scattered among the fifteen lysimeters according to a completely randomized design, threefold replicated.
The lysimeters were plastic-made tanks of about 1 m3 volume (0.95 m length, 1.15 m width, 1.00 m height), placed in galvanized steel frames, leaning on bricks displaced in two parallel rows at 0.50 m from the ground. Insulating panels surrounded the tanks, in order to decrease heat exchange between the soil and the air. The tanks were originally drilled on the bottom, then the hole was covered by a 5 cm thick layer of gravel in order to facilitate drainage. On the top of the gravel layer, a fine maze gauze sheet was laid down to filter out solids from drainage water and to avoid clogging. In 2010, the lysimeters were filled with soil with the following characteristics: 582 g.kg-1 of sand, 282 g.kg-1 of silt and 136 g.kg-1 of clay; 7.8 gkg-1 of organic matter (Nelson and Sommers, 1982); 8.5 pH; 0.45 gkg-1 of total N (Bremner and Mulvaney, 1982); 3.3 mgkg-1 of available P (Olsen et al., 1954). The hole at the bottom of each lysimeter was connected by a multilayer water pipe to a 30 L water tank, in order to allow drainage water storage, monitoring and sampling. 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128
The experiment was carried out at the “Enrico Avanzi” Centre for Agro-Environmental Research (CiRAA) of the University of Pisa, located in the Pisa coastal plain (43°40' N lat; 10°19' E long, with 1 m a.s.l. and 0% slope), Italy.
2.2 Agronomics
The experiment started in April 2011 with fertilization and transplanting of lettuce; after lettuce, chard and spinach were grown in sequence. One day before transplanting of each vegetable crop, fertilizers were mixed with the soil during seedbed preparation at 7 cm of depth, by applying 238.3, 230.7, 209.1 and 42.0 g f.m. per lysimeter, respectively for CDSM, PDSM, PDBM and AN. These amounts corresponded to 2181, 2112, 1914 and 384 kg f.m. ha-1, respectively for CDSM, PDSM, PDBM and AN, and in relation to total nitrogen contents of each fertilizer (Table 4) supplied each crop with 10.925 g of N per lysimeter (corresponding to 100 kg N ha-1). At these rates, the meals concentration in the top soil (7 cm of depth) varied from 1.9 to 2,2 ‰ (w/w); in the case of PDBM, the glucosinolate concentration in this soil layer was about 61.5 mg GLSs kg-1 of soil.
In order to evaluate the possible residual effect of the organic and mineral N fertilizers on N availability, an unfertilized barley crop was seeded after the spinach harvest, applying no fertilization. Barley harvesting was followed by a fallow period (from July to November 2012). During the crop cycles, weeds were manually removed. Further information about crop management are reported in Table 1.
Rainfall and temperature (maximum and minimum) were recorded during the experimental period and are summarized in Figure 1.
2.3. Sampling and analysis protocols
Samplings of crop biomass were performed at harvest maturity; all the plants present in each lysimeter were harvested, cutting them at the soil level. The biomass of each plant was split into marketable and unmarketable products, weighed just after harvesting and oven-dried at 60°C until constant weight was achieved. Nitric N was measured in the leaves of lettuce, chard and spinach by ionic chromatography (Dionex DX 120) (Camusso and Polesello, 1999). Total N concentration in the biomass of each crop was assessed using the Kieldhal method (Jones et al., 1991). After harvest, crop biomass was split into 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154
marketable produce (leaves for vegetables, grain for barley) and residues (unmarketable leaves for vegetables, straw for barley) in order to evaluate total N concentrations and uptakes.
Analysis on DOMs were focused on the determination of their residual oil, GLSs, total N, total P, organic carbon concentrations; for these analyses the following methods were used: the standard Soxhlet
extraction method using hexane as solvent was used for residual oil evaluation, GL content was determined following the ISO 9167-1 method (1992), with some minor changes of the extraction procedure (Lazzeri et al. 2011), total N and organic C were measured with the Kjeldhal method (Jones et al., 1991) and the Springer-Klee method (Gazzetta Ufficiale del 26/01/01 n.21, DM 21/12/00, Suppl. n.6), respectively. DOMs were also analyzed for Neutral Detergent Fiber (NDF was assessed using the filter bag technique Method 6) (Ankom Technology, 2011a), Acid Detergent Fiber (ADF was assessed using the filter bag technique Method 5) (Ankom Technology, 2011b) and Acid Detergent Lignin (ADL was assessed by the Method 8) (Ankom Technology, 2011c), that are parameters able to estimate the potential for water solubility of nutrients contained in the meals. In order to estimate the potential of the different meals in releasing nutrients, the meals were dissolved in water and shaken for 30 min and 120 min; at the end of the shaking period, aqueous extracts were passed through a cellulose acetate sieve of 0.2 µm. The extracts were analyzed for ammonium and nitrate using a Dionex DX 120 chromatograph equipped with an IonPac CS12 column for cation analysis and AS4A for anion analysis.
The sampling period for nitrate leaching monitoring started in March 2011, before the beginning of the experiment, applying 80 liters of water on the soil surface of each lysimeter in order to saturate the soil volume and remove residuals of N by leaching. After this pre-treatment, the water leached from each lysimeter was collected during the vegetable cultivation period (04.06.2011-12.23.2011) and the following year, during the barley cultivation and follow until 11.18.2012. The amount of water leached from each lysimeter and its nitrate concentration was measured whenever rainfall produced leaching. Nitrate concentration was measured using ionic chromatography (Dionex DX 120) (Mosello et al., 1998). 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178
N use efficiency was calculated according to the difference method proposed by Varvel and Peterson (1990), as the ratio of the difference between the fertilized crop uptake and the unfertilized crop uptake, and the amount of N applied to the crop.
In order to compute N balance, N inputs into the system were selected (N from fertilization, N from rainfall, N from irrigation, N from mineralization), as well as N outputs (N uptake by the aboveground biomass of crop and weeds, N leaching). N from rainfall was calculated multiplying rainfall amount of each cropping season (or period) by the average rainy water N concentration in the area (3 mg l-1) as suggested by Masoni et al., (2010). Irrigation water was analyzed for total N, but N concentration was instrumentally
undetectable. N from mineralization was calculated according to the model proposed by Masoni et al., (2010). Aboveground biomass N uptake and N leached were computed according to the previously described methodology.
2.4. Environmental assessment methodology
A GHG emission assessment by LCA was performed in vegetable crops, in order to compare fertilization effects by DOMs versus AN. The calculation of the chain emissions was carried out using the software So.Fi.A., developed by CRA-CIN (D’Avino et al., 2011a, 2011b), evaluating 100 year GWP due to carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) as the amount (mass unit) of CO2 equivalent (CO2eq). The following coefficients were used: 1 for CO2, 23 for CH4 and 296 for N2O, according to the methodology required by the European biofuel sustainability certification (Scarlat and Dallemand, 2011), taking into account the process inputs related to the production and use of fertilizers, pesticides, diesel and electricity for irrigation. Seedling input was not taken into account according to Romero-Gamez et al. (2014), Röös and Karlsson (2013) and Page et al. (2012), due to the local production of seedlings in not heated glasshouse. Direct and indirect N2O emissions from fertilizers and residues were calculated following IPCC (2006) tier 1, considering a reduction of 28% observed for solid organic fertilizer (Aguilera et al. 2013). N2O emissions from barley residues were not taken into account, since barley straw was generally totally removed from the field. All other input units were transformed into CO2eq, applying the coefficient reported in the JEC E3 database for biofuel sustainability certification, as suggested by the Harmonised Calculations of Biofuel 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204
GHGs in Europe (BioGrace, 2014), in order to reduce one of the main sources of variability of the final results (Reap et al., 2008). In this way, the use of values agreed at international level made it possible to compare the results, highlighting the differences in fertilization management. CDSM process yields were taken from Biograce version 4c (2014), as were PDSM yields, but modifying the extraction impact, reported in Biograce only for chemical extraction. The unique impact associated with mechanical extraction was 0.324 MJ of low voltage electric consumption per kg of pressing seeds measured in a small pressing plant; the decrease in oil extraction yields was also taken into account following Lazzeri et al. (2011). PDBM coefficients were calculated from Lazzeri et al. (2011), applying the B. carinata yield to sunflower impacts; inventory data to calculate DOM impact are summarized in Table 2. In fact, crop growing involves a series of burdens and these must be allocated equitably to the co-products, applying an allocation factor (Spugnoli et al., 2012). The main approach used was the economic value, because the causal relationship between the functions and the environmental burdens of raw material extraction and final waste management was economic instead of physical (Ekvall and Finnveden, 2001). The value applied was the three year (2011-2013) average price: 1.11 (±0.11 s.d.) € kg-1 sunflower oil (Indexmundi, 2014) and 0.20 (±0.04 s.d.) € kg-1 sunflower meal (Clal, 2014). No differences in sunflower oil or meal prices between mechanical and solvent extraction were observed in the commodity market. Values of B. carinata, as a minor crop, were linked to rapeseed stock exchange values (G. Patalano, personal communication), so the prices for oil and meals, similarly estimated, were 1.23 (±0.14) and 0.28 (±0.05) € kg-1, respectively. The impact associated with fertilization was calculated per kg of applied N, starting from these data and taking into account N content measured in DOMs.
Agricultural production inputs, as applied mimicking cultivation at open field scale, are described in Table 3. For each crop, the results are expressed (as functional units) per (i) hectare, (ii) kg of fresh marketable production and (iii) kg of total dry biomass.
2.5. Statistics
Analysis of variance (ANOVA) for a completely randomized design was performed using the Cohort CoStat Software (Monterey, CA, USA). Differences among treatment means were compared using a Fisher’s 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230
protected LSD test at P < 0.05 (Gomez and Gomez, 1984). Before analysis, the Bartlett’s test was performed to test the homogeneity of error variances. For dry matter (d.m.) content (lettuce), unmarketable yield (lettuce and spinach) and N concentration in total aboveground dry matter (barley), Bartlett’s test gave significant results, hence a proper transformation of data was applied (i.e. square-root transformation for data of d.m. content of lettuce and N concentration of barley, arc sine transformation for data of
unmarketable yield of spinach and lettuce).
3. Results and discussion
3.1. Defatted oil-seed meal characterization
The three defatted oilseed meals showed different fat concentrations in relation to the oil-extraction systems, but not in relation to the species (Table 4); differences in oil content may explain the poor
differences in organic carbon concentration being positively related to fat content. On the other hand, ash, N and total P concentrations were very similar among the meals and in agreement with the literature data (Moore, 2011). The percentages of NDF, ADF and ADL in the meals were lower for PDBM with respect to PDSM, as well as for PDSM in comparison with CDSM (Table 4). According to Cilliers and Cilliers (1995), these results suggest that PDBM is potentially more able to easily release nutrients in solution respect to PDSM and CDSM as confirmed by the higher relative abundance of NO3- and NH4+observed both after 30 min and 120 min in the water where PDBC was dissolved, respect to PDSM and CDSM (Table 4).
A significant difference between meals obtained from sunflower and from B. carinata is represented by the glucosinolate concentration: completely absent in the former and present at the concentration of 99.2 µm g-1 in the latter. The presence of GLSs characterizes the seeds from brassicaceae not genetically improved whose defatted meals may contain total GLSs at very high concentrations (> 200 µmol g-1 of defatted meal) (Handiseni et al., 2013). Because of the negative effect of GLSs on livestock health, a top limit in GLS concentration in defatted meals was imposed at 30 µmol GLSs g-1 of defatted meal (OECD, 2011). On the 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254
other hand, the presence of GLSs makes these meals useful agents against weeds, insect pests, nematodes and pathogens.
This ability is not selective and may negatively affect N mineralization and nitrification process in relation to the GLSs quality and concentration in the soil (Cabrera et al., 2005).
According to Bending and Lincoln (2000), 0.5 g 2-propenyl isothiocyanate g-1 of soil are able to reduce the nitrification process in a sandy-loam soil. Also Goos et al. (2009), using meal from Crambe abyssinica L., have observed a 35% inhibition of nitrification applying approximately 100 mg GLSs kg-1 of soil (in our study we have applied about 60 mg GLSs from B. carinata per kg of soil).
3.2. Crop productivity and quality
Lettuce - Ammonium nitrate application increased by 110% lettuce fresh marketable production with respect to control (Table 5). Sunflower DOMs (both completely and partially defatted) produced a similar effect but at lower level (+84% and +78%, respectively), while the PDBM increased marketable production by 44% compared to the control. Considering the total d.m. yield, the differences among treatments were poorer due to the higher d.m. content of the unfertilized lettuce at harvest. Similar results were obtained by Zaccardelli et al. (2008) studying the effects of sunflower and B. carinata meals on escarole. Nitrate
concentrations in leaf tissues did not exceed the reference values indicated by the EU (2011). In comparison to the control, lettuce fertilized with AN showed the highest nitrate concentration, while PDBM showed the lowest one among the fertilized crops (-60% related to AN).
Chard – The positive effect of mineral fertilization on chart marketable production was not different from that determined by organic fertilizers (Table 5). With respect to the control, fresh biomass production was increased by 194%, 196%, 188% and 153% respectively using AN, CDSM, PDSM and PDBM. Nitrate
concentration in chard leaves was not different among treatments, proving undetectable in all cases. Hence, chard response to mineral and organic fertilizers was very different with respect to lettuce response. Considering that chard uptakes were double the lettuce ones (Table 7), it is hard to ascribe the effects of the treatments to the lower chard N requirements; it is more likely that, beside the N applied at the beginning of its cycle, chard also used part of the residual organic N applied to lettuce about eighty days before. This 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280
highlights the capacity of the oilseed meals to satisfy crop nutrient requirements as well as mineral N fertilizers when periodically applied.
Spinach – Spinach response to mineral and organic fertilizers was similar to chard (Table 5). Ammonium nitrate application caused a 210% increase in fresh marketable spinach production with respect to control. CDSM and PDSM determined similar increments (227 and 231%, respectively); the effect of PDBM on spinach production was poorer with respect to the other meals (+158%). Spinach leaf nitrate concentration was not different among organically fertilized crops, but it was superior to the unfertilized spinach; using AN, the nitrate content in spinach leaves strongly increased up to 3700 ppm.
Barley - Barley was grown after spinach, without any fertilization in order to highlight the achievable residual effect on the subsequent crop of the fertilizers (organic and mineral) applied to vegetable crops (Table 6). AN applied to the vegetable crops the previous year had sustained the barley grain yield, making it possible to reach 5.5 t of dry grain per hectare (+ 168 % if compared to the control). The organic fertilizers also allowed barley to produce more than the control (+71, 66 and 56% for CDSM, PDSM and PDBM, respectively). No residual effect of mineral and organic fertilizers was observed on barley grain protein concentration: the lowest grain protein concentration observed in the case of AN application might have been due to the highest grain yield of the cereal.
On the whole, these findings indicate that the application of CDSM and PDSM on the selected vegetable crops had a similar impact to the application of AN, unlike PDBM that was not able to sustain crop production at the same level as the other meals. As supported by the poor residual effect of PDBM on barley, it is possible to hypothesize that PDBM was not able to release N so easily as both sunflower DOMs did.
3.3. Crop nitrogen concentration, uptake and use efficiency
N concentration in the above ground biomass of lettuce, chard and spinach was generally higher using AN, lower in the unfertilized crops and intermediate in the case of organic fertilizer application (Table 7). In the case of barley, differences among treatments were slight. The lowest N concentration in the biomass of 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305
barley was observed under the mineral fertilization system (AN) where the highest biomass production was observed.
A similar general trend was observed for N uptake. The crops fertilized with PDBM showed lower N uptakes if compared to PDSM (-4, -11, -12 and -10% for lettuce, chard, spinach and barley, respectively), confirming the hypothesis that in our experiment this kind of meal released N slowly with respect to sunflower meals. Nitrogen use efficiency (NUE) was poorly affected by the different types of organic fertilizers in any case, while the best N use efficiency was estimated for AN application. In comparison to lettuce, NUE values calculated for chard were, on average, three fold greater; in the same way, NUE values calculated for spinach were double the values observed for chard. Generally NUE values tended to be lower where PDBM was applied; only in case of spinach, where the residual effect of fertilizers applied to lettuce and chard cumulated, NUE under PDBM was significantly lower than under AN, while no differences were observed among PDSM, CDSM and AN, and likewise among the three defatted meals.
3.4. Nitrogen balance
N balance was analyzed for each crop cycle (lettuce: from 04.06.2011 to 06.03.2011; chard: from
06.27.2011 to 08.09.2011; spinach: from 09.28.2011 to 12.23.2011; barley: from 12.28.2011 to 06.28.2012; fallow: from 06.29.2012 to 11.16.2012) and for the first and the second year separately, because in the second year no fertilizer has been applied during the barley cycle and the fallow period (Table 8).
At the end of the lettuce crop cycle, the difference between N inputs (fertilizers, rainfall, mineralization) and N outputs (crop uptakes + leaching) indicated an appreciable N surplus in all the cropping systems: about 80-83 kg N ha-1 for the soil that received DOMs and 73 kg N ha-1 in the case of AN (Table 8). For chard, the N inputs exceeded N outputs, generating a surplus again. PDBM generated the highest surplus (53.6 kg N ha -1), while in the case of AN, the surplus was the lowest and was totally represented by mineral N (29.0 kg N
ha-1). The higher surplus calculated for PDBM suggests that this meal released N to chard at a lower rate in comparison to both sunflower DOMs. The N balance of spinach and the cumulated balance of the three crops in rotation at the end of the first year of research confirmed this hypothesis.
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Spinach N output was greater than input; as a consequence, the surplus was negative for all the systems except for PDBM. This likely means that: (i) part of the N surplus generated by lettuce and chard was used by spinach; (ii) PDBM reduced the N availability for chard in comparison to CDSM and PDSM.
At the end of the spinach crop cycle, considering the first cultivation period as a whole, AN fertilization increased the N mineral pools by 56.1 kg ha-1 while the organic fertilizer made it possible to increase the soil organic N pool by 99.5, 107.4 and 138.2 kg ha-1 for CDSM, PDSM and PDBM, respectively, showing an appreciable potential for sustaining soil fertility. It is noteworthy that the major accumulation resulted from application of B. carinata meal in comparison to sunflower meals confirms the hypotheses of a slower mineralization rate of this meal (Goos et al., 2009) and/or reduced growth and nitrogen uptake of vegetable crops due to inhibitory effects of GLSs present in B. carinata meal (Vaughan and Berlow, 1998).
At the end of the first study period, even though the amounts of leached N were poor, due to the low amount of rainfall, N leaching under AN was six fold greater than control. In this context, the use of organic fertilizer based on oilseed meals strongly reduced N leaching by 84-87%, leading to values to the same level as the unfertilized soil (control).
Considering the first experimental period as a whole (from April to November), the crops fertilized with sunflower meals used 67-64% of the N applied, with this percentage calculated as follows: [(Nrain + Nmineralization - Nuptake - Nleaching) / Nfertilizer] x 100. The crops fertilized with AN used, on the whole, 81% of the N applied in the first period (199 days) while those fertilized with PDBM only 54%. These results are similar to the results obtained by Moore et al. (2010) who, investigating the effect of mustard meal on nitrate release in an incubation experiment over 210 days, observed that the cumulative N total release was 61% for the total N in the case of B. carinata meal.
In the second year, the unfertilized barley caused a negative surplus, being higher under AN (-36 kg ha-1) and lower under PDBM (-12 kg ha-1). Barley was able to use 4-6% of the overall N applied the previous year as an organic fertilizer while it used 12% of the total N applied as AN in 2011.
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During the last experimental period, under bare soil, the amount of N produced by mineralization and deposited by rainfall exceeded the N uptake by weeds and the leached N, provoking a positive N balance in all the systems.
The N leached during the second experimental period under AN was similar to the normally estimated amount for the area and was significantly higher with respect to the other treatments (22.9 kg ha-1 vs 4.3 kg ha-1 under control, 4.0 kg ha-1 under CDSM and PDSM, 2.6 kg ha-1 under PDBM).
3.5. Environmental impact
The impact of meals was calculated sharing common GHG emissions of oil and meal by economy allocation: the allocation factor was lower in CDSM (19%) than PDSM and PDBM (30% and 35%, respectively), mainly due to the higher oil production per hectare (Table 2). Taking into account the N content in meal (Table 4), the GHG emissions of CDSM, PDSM, PDBM per kg of N were 3.7, 4.2 and 7.7 kg CO2eq, respectively. The relatively higher impact of PDBM was due to lower yield and a higher cultivation impact compared to sunflower standard values. The estimated impact of cultivation for sunflower (983 kg CO2eq ha-1) was considered low if compared to the one reported by Spugnoli et al. (2012) for standard sunflower
management in Mediterranean conditions. In any case, the values were comparable with standard values applied for emissions during production of synthetic fertilizers (5.9 kg CO2eq kg-1 N). In particular sunflower meals obtained a lower impact per kg of N, bearing in mind that organic N fertilizers have different
characteristics than synthetic N fertilizers, including lower NUE, slower nutrient release and an important organic matter supply.
The GWP of vegetable crop cultivation per area is reported in Figure 2a; the impact of these crops was mainly due to GHG emissions to produce energy for irrigation, followed by fertilization inputs and then by diesel oil for agricultural machineries. For these reasons chard, requiring 2440 m3 ha-1 of water, had highest impact, whereas barley, without irrigation and fertilization, had the lowest. Among treatments, unfertilized trials had the lowest impact per hectare, followed by trials fertilized with sunflower meals. The higher impact due to PDBM fertilization, in comparison with AN fertilization, was offset by lower N2O emissions estimated from fertilizers and from residues.
355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380
While the impact per area was obviously lower for unfertilized trials than fertilized, opposite results were observed using one kg of fresh marketable yield as a functional unit (Figure 2b), where unfertilized trials achieved a higher impact, due to lower yield. Conversely, the impact of sunflower meal treatments, that was lower per area unit compared to AN treatments, remained similar for all vegetable crops when computed per kg of fresh marketable yield, reaching only 38 g CO2eq per kg of fresh spinach in case of fertilization with CDSM. In barley, the high grain yield in lysimeters previously fertilized with AN (6.3 t f.m. ha-1 in AN vs 3.9 t f.m. ha-1 averaged over all DOM treatments) determined a lower GWP impact per kg of grain of barley. The residual effect of fertilization is outlined in Figure 2c, where impact per total dry biomass produced decreases in subsequent crops, confirming also the good results of sunflower meal treatments compared to AN treatments.
4. Conclusions
Partially defatted meals of sunflower and B. carinata and completed defatted meal of sunflower oilseed may be properly used as organic fertilizers for lettuce, chard and spinach, ensuring marketable yields comparable with those obtained using mineral N fertilizers (ammonium nitrate), especially when they are applied regularly within the crop rotation. Nevertheless, the study showed the lower efficacy of B. carinata meals in sustaining crop yield when compared to sunflower meals (both totally and partially defatted). In our
experiment, partially defatted B. carinata meal was always incorporated in the top soil (7 cm of depth) the day before crop transplanting. This technique might have produced a high concentration of GLSs in the soil layer explored by roots, concurrently determining a significant impact on soil biota and direct phytotoxicity phenomena. Plant transplanting, done immediately after fertilization with B. carinata meals, could have determined some phytotoxic effect of AITC (allyl isothiocyanate) release that generally occurs in the 48 hours after watering (Lazzeri et al., 2011).
The carbon footprint of B. carinata meals was also higher than the sunflower meal footprint; it is important to highlight, however, that in this study only the fertilization effects were assessed, while the added value of
B. carinata meals in replacement of conventional fumigants was not taken into account, and soilborne
381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406
pests, potentially controlled only by B. carinata meals, did not affect yields because trials were performed on soils with a low infestation of soilborne fungi and pests such as nematodes. This consideration could deeply reduce the impact of B. carinata meals (D’Avino et al., in preparation).
The residual effect of fertilization was shown, evaluating the reduced impact on GWP in subsequent crops. In terms of production quality, the application of oilseed meals reduced the nitrate concentration of nitrate in spinach leaves, an aspect that determines significant changes in food health.
From an agro-environmental point of view, in comparison to ammonium nitrate supply, the repeated application of the oilseed meals increased the organic N pool of the soil and reduced nitrate leaching to the same level as the unfertilized system.
Overall, our findings confirmed the high value of oilseed meals as a sustainable alternative to mineral fertilizers and an important nutrient source especially for organic crop production.
5. Acknowledgements
The trials were performed as part of the activities of the Project “Sistema Integrato di Tecnologie per la valorizzazione dei sottoprodotti della filiera del Biodiesel” (VALSO) financed by MiPAAF (D.M.
17533/7303/10 del 29/04/2010) and coordinated by CRA-CIN of Bologna.
The authors acknowledge all the staff of C.i.R.A.A. involved in the activities, in particular Massimo Sbrana, Elisabetta Moscheni and Rosalba Risaliti, who helped in management, sampling, chemical determinations and data analyses.
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Fig.1. Monthly mean minimum (Tmin), and maximum (Tmax) temperatures (°C), and total rainfall (mm) measured during the experimental period (2011-2012) and long-term averages (1993-2012).
Fig.2. Impact on global warming per: a) area unit (CO2eq per hectare); b) marketable product unit (CO2eq per kg of marketable produce fresh matter, f.m.); c) total biomass unit (CO2eq. per kg of total crop aboveground dry matter). Impact of subsequent vegetable crops grown in lysimeters, simulating cultivation in full-field. Crops were fertilized with 100 kg N ha-1 as completely (CDSM) and partially (PDSM) defatted sunflower meal, partially defatted Brassica carinata meal (PDBM) or ammonium nitrate (AN), or totally unfertilized (control). Barley, subsequently grown in the same lysimeters, was kept unfertilized. Salable yield counted for barley was grain expressed at commercial moisture (13% w/w). 598 599 600 601 602 603 604 605 606 607 608
