Iride Volpi1, Jonathan Trabucco1, Daniele Antichi2, Christian Frasconi2, Cristiano Tozzini1, Simona Bosco1*
1Institute of Life Sciences, Scuola Superiore Sant'Anna, via S. Cecilia, 3, 56127, Pisa
2 Dipartimento di Scienze Agrarie, Alimentari, e Agro-Alimentari, Università di Pisa, Via del Borghetto 80, 56124 Pisa
*s.bosco@santannapisa.it
Abstract
The integration of organic and conservation agriculture may help in improving soil quality, while reducing the use of agrochemicals. However, the impact of these practices on the emissions of greenhouse gases (GHGs) is not well defined, particularly in Mediterranean climates. A field-experiment was conducted to evaluate the effect of three systems, i.e. Integrated (INT), Organic (ORG) and Conservation Organic (ORG+), on a 2-year vegetable crop rotation (fennel, summer lettuce, cabbage, spring lettuce), on carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) emissions. The
monitoring of soil GHGs emissions was performed from October 2014 to July 2016, using an innovative instrument equipped with laser analyzers. Main results were that only CO2 emissions were affected by system management, while no
differences were observed in N2O and in CH4 emissions among treatments.
Keywords: soil flux; nitrous oxide; carbon dioxide; methane; no-till
Parole chiave: flussi dal suolo; protossido di azoto, anidride carbonica, metano, non lavorazione Introduction
Agricultural management practices affect the production of greenhouse gases (GHGs) from soil, especially nitrous oxide (N2O) and methane (CH4), which have global warming potentials, respectively, 265 and 28 times higher (without inclusion
of climate-carbon feedbacks) than carbon dioxide (CO2) (Myhre et al., 2013). Agricultural soils emit approximately 10.3–
12.8 Tg N2O-N year−1 (Butterbach-Bahl et al., 2013), as a result of two main biological processes, i.e. nitrification and
denitrification. CH4 is mainly consumed by methanotrophic bacteria in aerobic soil condition, while it may be produced by
methanogenic bacteria in anaerobic conditions (Hütsch, 1998). CO2 emissions from soils are mainly related to the
decomposition of soil organic matter (SOM), thus all the practices that increase soil C stock may mitigate CO2 emissions
(Paustian et al., 1997).
Conservation agriculture includes reduced tillage practices such as no tillage (NT) or minimum tillage (MT) and it may help in preserving or increasing SOM in soil (Holland et al., 2004). Moreover, conservation agriculture directly affects N2O
soil production even if with highly variable rate (Rochette et al., 2008). Organic/integrated farming systems are considered not compatible with conservation tillage mainly because of the reliance of conservative systems on herbicides to control increasing weed pressure in absence of inversion tillage, and on mineral fertilizers, to obtain satisfactory yields with a reduced mineralization rate (Peigné et al., 2007).
To achieve the integration of organic or integrated systems with conservative systems two major issues need to be addressed: the availability of specific machines for mechanical cover-crop management and physical weed control, and for sod-seeding/planting; and the optimization of cropping systems to improve nutrient cycling and minimize the development of weeds. Agricultural practices, such as N fertilization, tillage practices and cover cropping may enhance or mitigate GHGs emissions from soil, dependently on soil characteristics and environmental conditions (Smith et al., 2008). However, information on GHGs emissions from crop rotation under conservative agriculture is scarce and contradictory results were reported (Tellez-Rio et al., 2017).
The "Smart Management of Organic Conservative Agriculture" (SMOCA) project (2014-2017) aimed at: i) designing an innovative cropping systems able to integrate organic agriculture and conservative agriculture; ii) developing innovative machines; iii) comparing three systems; integrated (INT), organic (ORG) and a conservative organic (ORG+) systems; considering the agronomic performances, the environmental impacts and economic point of view. In particular, the aim of this paper is to present the effect of the three management systems on CO2, CH4 and N2O.
Materials and Methods
The study was conducted in the Pisa coastal plain (43° 40' N Lat; 10° 19' E Long; 1 m a.s.l and 0% slope), at the “Enrico Avanzi” Centre for Agro-Environmental Research of the University of Pisa. The climate is typical Mediterranean, characterized by a long-term average annual rainfall of about 900 mm and a mean annual temperature of 15°C (1986- 2013). The field trials consisted in a 2-year vegetable crop rotation, cultivated under three different management systems
managed according to: integrated farming recommendations with conventional tillage practices and mineral fertilization (INT); organic farming regulations with conventional tillage practices, organic fertilizers, green manures and mechanical weed management (ORG); organic farming regulations integrated with conservative practices including no-tillage, organic fertilizers and physical weed management (ORG+). The rotation was replicated both in space and time, thus each crop was present every year. The crops included in the rotation were: savoy cabbage (Brassica oleracea var. sabauda L. cv. Famosa), spring lettuce (Lactuca sativa L. cv. Justine), fennel (Foeniculum vulgare Mill. Cv. Montebianco) and summer lettuce (cv. Ballerina). ORG included a spring green manure mixture incorporated into the soil before transplanting of summer lettuce, composed by field pea (Pisum sativum L.) and faba bean (Vicia faba subsp. minor L.), and a summer green manure mixture, incorporated into the soil before fennel transplanting, composed by red cowpea (Vigna unguiculata L.), buckwheat (Fagopyrum esculentum L.), millet (Panicum miliaceum L.) and foxtail millet (Setaria italica L.). ORG+ included a summer green manure, terminated as dead mulch by roller crimper and flaming before transplanting of fennel, composed by the same plants used in the spring green manure of ORG, and a red clover (Trifolium pratense L.) directly seeded and established as a living mulch for summer lettuce and cabbage.
N fertilizer rate (kg N ha-1) Fennel Summer lettuce Cabbage Spring lettuce INT 122 46 108 27 ORG 77 0 59 20 ORG+ 28 0 28 0
The rotation was replicated in two adjacent fields and for each field the three systems were randomized with three replicates each.
The monitoring of soil GHGs emissions was performed from October 2014 to July 2016, using an innovative instrument developed within the LIFE+ project “Improved flux Prototypes for N2O emission from Agriculture” (IPNOA, www.ipnoa.eu) and though the flow-through non-steady state chamber. The instrument was a light tracked vehicle that can be operated by remote control and equipped with an Ultraportable Greenhouse Gas Analyser (UGGA) to measure CO2,
CH4 and water vapour, and a N2O, carbon monoxide (CO) and water vapour detector that uses off-axis integrated cavity
output spectroscopy (ICOS), both provided by Los Gatos Research (LGR) Inc. (Mountain View, CA, USA). Output gas concentrations are given with a scan rate of 1 s. Measured data were recorded by a smartphone connected via Bluetooth®. Technical details on the instrument and its validation are reported by Bosco et al. (2015) and Laville et al. (2015). A PVC collar (15 cm height, 30 cm ø) was inserted to 5 cm soil depth in each plot within plant rows. Daily flux of each GHG was calculated from the slope of the linear increment of the gas concentration within the chamber plus collar volume during the chamber deployment time (2-3 minutes), considering the mean daily atmospheric pressure, mean daily temperature and the ratio headspace volume/area of the chamber.
Results and Discussion
CUMULATIVE GHGS EMISSIONS
Cumulative CO2 emissions in the two years resulted to be significantly lower in INT (30 ±2 t CO2-C ha-1) than in ORG and
ORG+ (37 ±2 t CO2-C ha-1 and41 ±2 t CO2-C ha-1, respectively). The higher soil respiration in ORG could be explained
with more frequent field operations for weed management and higher organic inputs (green manure) that might have enhanced soil carbon mineralization (Sanz-Cobena et al., 2014). The highest level for ORG+ may be due to the mineralization of higher organic inputs. However, a partition of soil respiration in autotrophic and heterotrophic component would be necessary to understand the main sources of CO2 from soil.
No differences were observed among the three systems for cumulative N2O emissions, considering the whole monitoring
period (crop growing periods and fallow periods) and they were on average 10 ±3 kg N2O-N ha-1. Surprisingly, the lower
nitrogen fertilization in ORG and ORG+ did not achieve to reduce N2O emissions. This could be explained by an increment
in N2O emissions due to green manure mineralization (Sanz-Cobena et al., 2014), especially considering legume crops,
both in ORG and ORG+ systems and a progressive increase in water filled pore space (WFPS) that can promote N2O
production (Smith et al., 2008).
Similarly, cumulative CH4 emissions were not different among the three systems and they were on average equal to -0.7
±0.2 kg CH4-C ha-1. Thus, cumulative CH4 emissions resulted in a net uptake in the three systems, in agreement with what
reported by some authors in Mediterranean environment (Sanz-Cobena, et al. 2014; Tellez-Rio et al., 2017), even if other studies reported that an increased tillage intensity or N fertilizer rate may decrease CH4 uptake in soil (Le Mer and Roger,
2001; Ussiri et al., 2009).
Tab. 1: N fertilizer rate in the three systems for each crop included in the rotation.
Tab. 1: Dose di azoto distribuita con la fertilizzazione nei tre sistemi per ogni coltura inserita nella rotazione.
Fig. 1: Average cumulative a) N2O (kg N2O-N ha-1), b) CO2 (t CO2-C ha-1) and c) CH4 (kg CH4-C ha-1) emissions in the
three systems, along the whole monitoring period (642 days).
Fig. 1: Media delle emissioni cumulate di a) N2O (kg N2O-N ha-1), b) CO2 (t CO2-C ha-1) e c) CH4 (kg CH4-C ha-1) nei tre
sistemi, considerando l’intero periodo di monitoraggio (642 giorni).
Conclusions
The comparison of integrated, organic and conservation organic was tested in a vegetable rotation on GHG emissions from soil. The results from the first two years showed that nor ORG nor ORG+ achieve to mitigate N2O flux from soil, while
they showed higher CO2 flux than INT. However, only a part of this flux can be related to carbon mineralization, indeed a
CO2 flux partition study would be needed to evaluate the different sources of CO2. However, all the three systems were
identified as net sinks of methane. Long-term studies are needed to verified the effect of the tested systems on GHGs flux from soil.
Acknowledgments
This research was funded by the Ministry of Education, University and Research - MIUR, FIRB 2013. Project title: Smart Management of Organic Conservative Agriculture - SMOCA (2014-2017) (http://smoca.agr.unipi.it/). The authors would like to acknowledge Fabio Taccini for the support in conducting the field trials.
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