Leonardo Verdi1*, Marco Napoli1, Marco Mancini2, Mirjana Ljubojević3, Anna Dalla Marta1, Simone Orlandini1 1 Dipartimento di Scienze delle Produzioni Agroalimentari e dell’Ambiente, Università di Firenze, Piazzale delle Cascine 18, 50144, Firenze, Italy 2 Fondazione per il Clima e la Sostenibilità, via Giovanni Caproni 8, 50145 Firenze, Italy
3 Department for Fruit Growing, Viticulture, Horticulture and Landscape Architecture, Faculty of Agriculture, Trg Dositeja Obradovića 8, Novi Sad, Serbia
*leonardo.verdi@unifi.it
Abstract
Emissions from the use of fertilizers represent a serious issue for the sustainability of agricultural systems, also considering the increasing of human population and the world food demand. In the present experiment, we evaluated the role of soil organic matter on the emissions of greenhouse gasses after the application of different fertilizers as liquid fraction of digestate from pig slurries, compost from organic fraction of municipal solid wastes, and urea on bare soil in order to avoid the influence of the crop. The experiment was performed on twenty-four bare soil pots with two levels of organic matter (low and high). Emissions were directly monitored through the use of a static chamber system and a portable gas analyser. Results show that soil organic matter as well as the composition of the fertilizers affect greenhouse gasses emissions. In particular, after fertilization we observed an increase of greenhouse gasses emission both for organic and chemical fertilizers. An exception is represented by methane emissions that show higher values in correspondence of lower soil organic matter content. This is probably due to temporary anaerobic conditions caused by soil saturation and compaction observed after digestate (liquid) and compost (dry and milled) application. This, together with high levels of easily degradable carbohydrates and easily accessible nitrates, provides conditions for an overabundance in the denitrifier community, especially in the first 72h (from day 1 to 3).
Keywords: static chambers, nitrogen, fertilization management, sustainability Parole chiave: camere statiche, azoto, fertilizzazione, sostenibilità
Introduction
Intensive agricultural systems in both plant and livestock production drastically increased the concentration of greenhouse gasses (GHG) as carbon dioxide (CO2) methane (CH4) and nitrous oxide (N2O), as well as ammonia (NH3) as the precursor
of N2O and main nitrogen loss trough volatilization. Deforestation and land use change for agricultural purposes led not just
to the air CO2 level increase, but also higher CH4, N2O and NH3 atmospheric concentrations due to agricultural activities,
including the overabundant and/or inappropriate use of fertilizers. It is well known that agricultural food-producing systems currently contribute to one-third of total anthropogenic GHG emissions that are assumed to increase with the increasing demand for agricultural products (Carlson et al., 2014). In order to keep such systems sustainable, future research must be focused on economically cost–efficient as well as environmental–friendly measures to mitigate climate gas emissions from agriculture. Pertinent literature addresses the emissions and solutions regarding different in situ soil and management conditions (Henault et al., 1998), application techniques (Carozzi et al., 2013), as well as fertilizer and manure type in laboratory conditions (Velthof et al., 2003). To the author’s best knowledge there is a scarce literature regarding GHG emissions from in situ bare soil, addressing both the emissions and possible control of the major gases. Setting the experiment on bare soil allows to define its basic emission potential independently from the crop, enabling the expression of organic matter as one of the main factors affecting emissions from the soil, and derivation source.
Based on these considerations, the main goal of the study was to evaluate the role of the soil organic matter content on gasses emissions after fertilization with different fertilizers, including liquid fraction of digestate from pig slurries, compost from organic fraction of municipal solid waste (OFMSW) and urea, as mineral fertilizer.
Materials and Methods
The experiment was conducted on 9.5 litres pots, placed in the open field and exposed to the environmental conditions. Each pot was filled with 8 kg of a silty-clay soil from experimental fields of CREA-ABP located in Scarperia, Firenze (43°58’56” N, 11°20’53” E). The experiment was set on the bare soil in order to investigate the sole role of the organic matter without any crop interference. The soil was taken from the depth of 30 cm, including top and sub soil layer, mixed before filling the pots, in order to homogenise it. The experimental design consisted of two contrasting levels of soil organic matter – OM1 1.3% and OM2 4.3% - with four treatments. Treatments included two types of organic fertilizers (liquid fraction of digestate from pig slurries and compost from organic fraction of municipal solid waste) as well as one mineral fertilizer (urea), with
the non-fertilized pots as control treatment. The digestate was produced by ‘Fattoria di Corte Marchesi De' Frescobaldi’ farm (Florence, Italy, 43°58’29” N, 11°23’21” E), while the compost derived from composting plant of ‘Alia Servizi Ambientali Spa’ (Florence, Italy, 43°55’580.95’’ N, 11°21’00.09’’ E). The amount of each fertiliser varied according to its N content (Table 1), calculated on the base of apre-defined quantityof 150 kg N/ha. GHG emission measurements were conducted three times in the first week (immediately after fertilization, after 48h and 96h) and once in the second week in order to investigate the emission trend. Experimental pots remained opened between the successive measurements to enable nitrogen volatilization, as these conditions would be the closest to the ones occurring naturally. CO2, CH4, N2O and NH3 emission
rates were measured by means of a static chambers system (Parkin and Ventera, 2010) and a portable gas analyser XCGM 400 that use NDIR technology for CO2, CH4 and N2O analysis and electrochemical technology for NH3. Temperature inside
the chambers and meteorological data (air temperature, wind speed and precipitation) were monitored continuously by thermocouples and an automatic meteorological station, respectively. Except for the first day, two hours prior to the measurements 10 mm of water were added to each pot for accelerating the beginning of the emissions process.
Table 1: Nitrogen content of three fertilizers Tabella 1: Contenuto di azoto nei tre fertilizzanti
Urea Digestate Compost N content Total % 46 0.319 2.27
N-NH4+ % - 0.284 0.15
N-NO3- % - 0.035 0.0013
Results and Discussion
In the present experiment, we investigated the role of soil organic matter on GHG emissions due to organic and mineral fertilizers applied on bare soil. Emissions data from each test are summarized in Table 2.
CO2 EMISSIONS
Results show that an enrichment of soil OM content positively affects CO2 emissions. As affirmed by Paustian et al. (2000)
and Six (1999), CO2 emissions dynamics from agricultural soil are affected by a wide range of factors. In this respect, OM
represents one of the main factors due to its influence on soil respiration. In the present study, a higher soil OM probably increased soil respiration and consequently CO2 emissions. Organic fertilizers (digestate and compost) produced higher
emissions compared to urea. In particular, the highest emissions were registered for digestate also due to its composition, rich in water, that allows the infiltration into the soil. An enrichment of water content of soil combined to the mild air temperatures occurred probably encouraged the proliferation of soil microorganisms and consequentially soil respiration. However, we did not observe significant differences in digestate emissions behavior between OM1 and OM2.
Urea produced a higher level of CO2 compared to compost, and the role of OM is evident. In fact, cumulative CO2 emissions
in OM2 were more than 3 times higher than in OM1.
CH4 EMISSIONS
Results obtained from organic fertilizers (digestate and compost) showed that CH4 had an opposite trend compared the others
gasses monitored. In particular, organic fertilizers produced more emissions in OM1 than in OM2. As described by Le Mer and Roger (2001) also CH4 emissions from soil are affected by many factors and in particular, a negative correlation between
CH4 emissions and C/N ratio was reported. In this respect, the composition of manure used to obtain the two levels of OM,
which represent the 25% of total organic C, can partially explain the behavior of CH4 emissions from organic fertilizers. In
addition, the composition of organic fertilizers, rich in total organic C (34.5% and 25.6% for digestate and compost, respectively), may have reduced CH4 emissions. In the case of urea, that did not provide organic C, the positive correlation
between OM level and CH4 emissions was confirmed.
N2O EMISSIONS
From the observed results we can affirm that N2O emissions are positively affected by the OM content of soil. For all tested
fertilizers N2O emissions in OM2 were higher than in OM1. In particular, digestate produced the highest emissions and this
is probably due to its high water content that determine anaerobic conditions with consequent greater N2O losses compared
to the others fertilizers. Moreover, the higher amount of organic C available into the soil in OM2 probably encouraged the microorganisms activity and N degradation. Also, the high rate of readily available N compounds of digestate and the warm temperature occurred during the experiment (average of 28.4°) enhanced N losses in the first two weeks after fertilization. On the other hand, compost emitted a N2O rate comparable with the control, probably due to its low water content. This result
is in accordance with the findings of Dalal et al. (2010), confirming that the application of compost can be considered an efficient strategy to reduce N2O emissions. Finally, concerning urea, its low water content reduces the risk of anaerobic
NH3 EMISSIONS
NH3 emissions were nearly five times higher in correspondence of OM2 than OM1 treated with urea. Again, this confirms
that higher organic C content into the soil modifies the C/N ratio and encourages bacteria activity with greater degradation of N and NH3 losses.
Organic fertilizers are an exception: digestate, in fact, showed the highest rate of NH3 emissions. However, we observed no
differences between emissions in the two OM levels. As digestate, also compost show comparable NH3 emissions. This fact
may mean that OM content of soil does not affect NH3 volatilization dynamics.
Table 2: Gas emissions from each treatment for OM1 (1.3% organic matter content of soil) and OM2 (4.3% organic matter content of soil 1% organic matter content of soil) expressed as kg C or N per hectare in 26 days’ period.
Tabella 2: Emissioni di gas nei diversi trattamenti per OM1 (contenuto di sostanza organica del suolo pari a 1.3%) e OM2 (contenuto di sostanza organica del suolo pari a 4.3%) espressi in kg di C o N per ettaro in un periodo di 26 giorni
CO2 CH4 N2O NH3
OM1 OM2 OM1 OM2 OM1 OM2 OM1 OM2
No-fertilizer
38,50
129,19
8.06 8.06 0.04 0.31 0.00 0.06Digestate
604,12
679,75
15.07 12.65 0.96 7.65 0.61 0.59Urea
67,04
206,67
8.95 11.17 0.09 0.29 0.09 1.15Compost
29,22
169,35
9.62 8.38 0.03 0.38 0.26 0.54Conclusions
This experiment was performed to evaluate the role of soil organic matter on the gas emissions that occur from soil after fertilization. The study focused on bare soil, allowing to understand the emissions dynamics without the influence of the plants. Based on the results, we observed that organic matter plays a key role on the emissions of GHG, generally
enhancing the levels of gas emissions. However, results about CH4 emissions of digestate and compost, which were higher
in OM1 than in OM2, require further investigation with particular attention to the role of microorganisms population.
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