Root exudate carbon mitigates nitrogen loss in a semi-arid soil

Abstract

C mineralisation Immobilisation N:I index 15 N isotopic pool dilution Nitrification Nitrous oxide a b s t r a c tThe need for increased food production to support the growing global population requires more efficient nutrient management and prevention of nitrogen (N) losses from both applied fertiliser and organic matter (OM) decomposition. This is particularly important in semi-arid rainfed cropping soils, where soil water and temperature are the dominant drivers of N cycling rather than agricultural management. Here we used 14 C and 15 N techniques to examine how peptide/amino acid turnover, gross and net N transformation rates and nitrous oxide (N 2 O) emissions responded to long-term plant residue additions and/ or short-term root exudate additions. Soil was collected from a semi-arid rainfed field trial with one winter crop per year followed by a summer fallow period, where additional inputs of straw/chaff over 10 years had increased total soil organic C (SOC) by 76% compared to no extra additions (control). These field soils were incubated in the laboratory with or without a synthetic root exudate mixture at a range of temperatures reflecting regional field conditions (5e50 C). Long-term plant residue additions (to build up total soil OM) did not decrease the risk of N loss as defined by the nitrification:immobilisation (N:I) ratio at most temperatures, so was not an effective management tool to control N losses. In comparison, short-term root exudate additions decreased the risk of N loss at all temperatures in both the control and plant residue treatment field soils. Increased net N mineralisation and decreased microbial C use efficiency at temperatures greater than 30 C resulted in significant ammonium (NH þ 4 ) accumulation. Microbial decomposers appeared to use amino acid-C for growth but peptide-C for energy production. Findings indicate that the greatest risk of N loss in these semi-arid soils will occur during rains at the start of the growing season, due to inorganic N accumulation over summer fallow when there are high soil temperatures, occasional significant rainfall events and no growing plants to release root exudates. While most attempts to manipulate the soil N cycle have occurred during the winter cropping period, our findings highlight the need to manage N supply during summer fallow if we are to minimise losses to the environment from semi-arid soils. Crown

N2O emissions from grain cropping systems: a meta-analysis of the impacts of fertilizer-based and ecologically-based nutrient management strategies

“…Previous field and laboratory studies have reported such relationships under similar environmental conditions (Huang et al 2004;Gomes et al 2009). The fact that this relationship emerged despite environmental variation further highlights the importance of C abundance in regulating N 2 O emissions and supports the idea that C and N should be managed in conjunction with one another in order to improve N use efficiency and N retention (Drinkwater and Snapp 2007;Fisk et al 2015).…”

“…The N saturation conceptual model highlights the significance of C-N coupling in driving N retention/loss and has led to major advances in C and N, and more recently, P biogeochemistry (Fenn et al 1998;Luo et al 2004;Magnani et al 2007;De Schrijver et al 2008;Mulholland et al 2008;Schlesinger 2009;Fenn et al 2010;Vitousek et al 2010;Crowley et al 2012). Application of this conceptual framework could have similar ramifications for N management in agricultural systems (Drinkwater and Snapp 2007;Thorburn et al 2011;Fisk et al 2015;Attard et al 2016;Tosti et al 2016).…”

“…Recoupling C and N can foster microbially-mediated processes that enhance internal N cycling pathways that favor N retention/N accrual (Fisk et al 2015). Previous meta-analyses have shown that using ENM practices increased N retention and reduced NO 3 -leaching compared to practices that only target improved crop assimilation of added fertilizer (Tonitto et al 2006;Gardner and Drinkwater 2009;Quemada et al 2013) suggesting that ENM can be used in conjunction with the 4Rs to optimize N fertilizer management and reduce environmental N losses.…”

Plant-Mediated Changes in Soil N-Cycling Genes during Revegetation of Copper Mine Tailings

“…In the paddy and forest soil, the denitrification was unapparent under neutral aerobic (Lan et al, 2015;Pajares and Bohannan, 2016) and/or nitrogen deficiency conditions (Pajares and Bohannan, 2016). In addition, root exudate carbon was previously observed to be effective at decreasing the risk of N loss by increasing the potential for heterotrophic microbial immobilization (Fisk et al, 2015) and plant roots might not provide an appropriate survival environment for denitrifiers (Fisk et al, 2015). Under aerobic conditions, the nitrogen loss was mainly attributable to ammonia oxidizers (Florio et al, 2016), and in this study the stimulating effects of plant roots on ammonia oxidizers still did not aid the accumulation of nitrogen.…”

A Newly Practice to Mitigate N2O Emission from Winter Wheat Soil by Intercropping Isatis indigotica

“…This was the first report of new land use pattern by co-cropping herbal plant to mitigate N 2 O emitted from agricultural soil. Compared to some other agricultural practices such as N fertilizer management (KAISER et al, 1998;Ulrike et al, 2003;Chu et al, 2007;Fub et al, 2011;Aguilera et al, 2013;Fisk et al, 2015;Frederico et al, 2015), reduced tillage (Fub et al, 2011;Fisk et al, 2015;Frederico et al, 2015), drip irrigation and fertigation (Aguilera et al, 2013;Kennedy et al, 2013), organic amendments (Qiu et al, 2013;Aguilera et al, 2013;Fisk et al, 2015), crops straw incorporation (Ma et al, 2009;Yang et al, 2015), biochar (Xiang et al, 2015), nitrification inhibitor (Ranucci et al, 2011;Barneze et al, 2015), this newly developed approach could be more effective and economical as it was simple and easily to conduct in a low-cost manner for farmer. Of course, it needs to be further improved and optimized.…”