Aggregate-associated N and global warming potential of conservation agriculture-based cropping of maize-wheat system in the north-western Indo-Gangetic Plains

Despite conservation agriculture (CA) is being promoted as a climate resilient technology, limited information is available on its impacts on N storage within soil aggregates vis a vis global warming potential (GWP) under tropical agro-ecosystems. Hence, this study assessed the effects of a medium-term (5-years) CA on total soil N (TSN) changes in bulk soils and aggregates, N2O and CO2emission, GWP and total C fixed in soils under maize (Zea mays L.)- wheat (Triticum aestivum L.) system on the Indo-Gangetic Plains (IGP). The treatments were: conventional tillage (CT), zero tillage (ZT) with planting on permanent narrow beds (PNB), PNB with residue (PNB + R), ZT with planting on permanent broad beds (PBB), PBB with residue (PBB + R), ZT on flat land/plains without crop residue (ZT) and with crop residue retention (ZR + R). Soil samples were collected after five years of a maize-wheat system and TSN in bulk soils and their aggregates of the 0-5 and 5-15 cm soil layers were measured along with N2O and CO2 emissions during the fifth year (2014-15).The soils under PBB + Rhad 37 and 9% more macro-aggregate-and micro-aggregate-associated N concentrations in topsoil (0-5 cm layer) than CT (248 and 299 kg N ha−1). However, topsoil soil aggregation and aggregate-associated N contents of PNB + R and ZT + R were similar to CT plots. The dehydrogenase and fluorescein diacetate activities and TSN, microbial biomass N, NO3-N and NH4-N concentrations were also highest in PBB + R plots in topsoil. The topsoil dehydrogenase activity was significantly correlated (r = 0.426, n = 21, p < 0.05) with CO2emission and with N2O emission (r = 0.770, n = 21, P < 0.01) during wheat (2014-15). However, topsoil FDA activities and MBN concentrations were only significantly correlated with N2O emission in wheat. In the maize-wheat system, highest N2O emission was observed in PNB + R plots and least in CT plots. But, PBB + R and PNB + R plots had similar CO2 emissions to CT plots in both crops. Despite GWP of ZT + R and PBB + R plots in the maize-wheat system were ∼5% higher than CT, greenhouse gas (GHG) intensities in the CT, PBB + R and ZT + R plots were similar. Thus, PBB + R practice is a better management alternative for soil N improvement (and a reduced fertilizer N dose could be adopted in future) than CT since this practice also had 36% and 8.2% higher biomass productivities of maize and wheat, respectively in the maize-wheat cropping system and similar GHG intensity to CT plots.