Impact of rainfall to the effectiveness of pig slurry shallow injection method for NH3 mitigation in a Mediterranean soil
Introduction
The need to increase food production in response to population growth and changes in diet has led to an important growth in livestock production systems (Springmann et al., 2018). Large amounts of animal manure are being produced as a result of this (Oenema et al., 2005; Steinfeld et al., 2006; Recio et al., 2018). In Europe, more than half of the total nitrogen (N) excreted is applied to croplands (Oenema et al., 2007). If properly done (i.e. considering N crop needs), partial substitution of synthetic fertilizers by organic ones can give satisfactory crop yields while closing loops within the croplands-livestock systems continuum (e.g. Guardia et al., 2017; Sánchez-Martín et al., 2017). Additionally, the application of manures to agricultural soils may enhance soil organic matter, generally low in semiarid regions under (e.g.) Mediterranean conditions (Aguilera et al., 2013; Sanz-Cobena et al., 2017a). However, these potential benefits of recycling and reusing animal wastes as fertilizers could be negated by poor manure management (e.g. broadcast application). The N use efficiency (NUE) of manures is generally low, i.e. only 20–52% of the excreted N is recovered by crops (Oenema et al., 2007; Ma et al., 2010), with most of the N being lost to the atmosphere mainly as ammonia gas (NH3). Globally, about 90% of NH3 emissions are from the agricultural sector (Bittman et al., 2014), with 64–75% arising from livestock production, including manure management and application to farmlands (Steinfeld et al., 2006; Webb et al., 2005). Adverse environmental, economic and public-health impacts associated with NH3 emissions (and deposition) have led to the development of legislative and technical initiatives to face such socio-environmental issues (Van Grinsven et al., 2013; Sutton et al., 2011; Bittman et al., 2014). Emission reduction targets for 2030 have been established through the UNECE Gothenburg Protocol (UNECE, 1999) and the recently revised EU National Emissions Ceilings Directive (EEA, 1999). In Spain, increasing use of synthetic fertilizers and, mostly, a national growing pig industry (3rd major exporter of pig products worldwide; MAPA, 2018) have led to an increasing trend in NH3 emissions over recent decades. Emissions in 2015 (most recent published data) were 476.2 kt, 5% higher than for 2014 (MAPAMA, 2017). Under international commitments, Spain has a national emission ceiling of 353 kt for the year 2030, requiring a 26% reduction in emissions compared with current levels. Therefore, abatement measures need to be urgently implemented (MAPAMA, 2018).
Low emission slurry application techniques represent one of the key technical strategies that have been included in national and regional programmes for NH3 pollution abatement (Sommer et al., 2001; Huijsmans et al., 2003; Sanz-Cobena et al., 2014a,b). Research in several countries has demonstrated that the injection and incorporation of slurry, compared with surface broadcast application, can reduce NH3 volatilization by up to 90% due to the reduction in the contact surface between the atmosphere and the ammonium N applied with the manure (e.g. Huijsmans et al., 2003; Recio et al., 2018). Nevertheless, although effective in abating volatilized NH3, the injection of slurries may have associated side-effects through the release of other N compounds to the environment (e.g. N2O, NOx, NO3−) (Sanz-Cobena et al., 2017b). Added to the enhanced availability of NH4+ at the soil surface, the use of slurries as organic amendments increases both soil moisture and easily degradable C forms, thus potentially leading to an increase in heterotrophic activity. Additionally, aeration and gas diffusivity in the soil can be temporarily reduced due to changes in soil porosity and an enhancement of soil respiration, thus promoting anaerobic microsites where stimulation of N2O emissions via denitrification as well as a decrease in CH4 oxidation may occur (Chadwick et al., 2000; Akiyama et al., 2004; Meijide et al., 2009; Webb et al., 2014). Apart from manure characteristics, the magnitude of both NH3 emissions and the associated environmental trade-offs following application to farmlands (e.g. GHG emissions) are highly dependent on local soil and weather conditions (e.g. wind speed and temperature) (Huijsmans et al., 2003). Therefore, the assessment of any abatement strategy needs to be carried out in a regional context (Sanz-Cobena et al., 2014a,b; 2017b).
A field experiment was carried out aiming to evaluate the effect of two contrasting pig manure application techniques (i.e. surface broadcast and shallow injection) on NH3 volatilization from a fertilized soil in Central Spain. The soil had not received any N application within the 10 years preceding period. Ammonia volatilization was quantified by the micrometeorological Integrated Horizontal Flux (IHF) technique across two contrasting campaigns in terms of soil moisture (i.e. saturated and non-saturated soil, WFPS≥60% for saturated soils). This enabled us to assess the impact of site-specific environmental conditions on the NH3 abatement potential of shallow injection. Additionally, the side effects of this management practice in the form of GHG fluxes (N2O, CH4 and soil respiration) were assessed under highly denitrifying conditions triggered by the increased soil moisture.
Section snippets
Experimental location and slurry application
The study was conducted in a Typic Xerofluvent soil at “La Poveda” field station in Madrid (40° 18′ 14'' N; 3° 25′ 57'' W) within a fallow period between two consecutive barley (Hordeum vulgare) crops. Some relevant characteristics of the top soil layer (0–20 cm), measured by standard methods of soil analysis (Soil Survey Laboratory Methods Manual, 2004) were: total organic matter, 1.4%; pHH2O, 8.1; bulk density, 1.47 Mg m−3; CaCO3, 3.4%; field capacity, 20.2% (w/w); porosity, 46%; sand, 37%;
Weather conditions and soil moisture
Average soil temperature was similar for the two campaigns (13.9 and 15.1 °C, respectively) but there were large differences in rainfall and consequently soil moisture. Average wind speed was 1.3 and 4.1 m s−1 in Campaign I and II, respectively. Cumulative rainfall over the 12 days following slurry application in Campaign I was 48.2 mm, with 25.4 mm within the first 48 h. In contrast, total rainfall over the 12 days in Campaign II was 0.6 mm. Average WFPS in Campaign I was 70%, ranging from 54
Influence of soil moisture on the efficacy of slurry injection as an NH3 abatement technique
Volatilized NH3 was highly affected by local edafo-climatic conditions, particularly intense rainfall monitored in campaign I at the application time (Fig. 2). Rainfall would have had a washing effect on SPS increasing transfer into the soil. The effect of rainfall on reducing emissions was greater for surface application than for injection – therefore the reduction potential of injection was less under rainy conditions. Contrasting effects of simulated rainfall on the effectiveness of manure
Conclusion
This field study is among the first to asses, under real conditions and using the micrometeorological IHF measurement technique, the NH3 abating effect of shallow injection of PS in a Mediterranean cropping system. The novelty and utility of the generated data is highly valuable, especially for national NH3 emission inventory development in a country with mandatory commitments to decrease these emissions. Nevertheless, due to the very limited extent of data generated here, we argue that more
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Authors of this research are grateful to the personnel at la Poveda field station (CSIC), as well as to Ana Ros, Paloma Martín and Pilar Ortiz because their technical assistance at the laboratory. We would like to recognize also the work of Rosa Loma, Ma Carmen Blanco, María Arróniz, Esperanza Luque and Katerina Kucerova due to their invaluable skills in the management of research projects at ETSIAAB and CEIGRAM (UPM). This research has been possible thanks to the economic support of regional
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