Four years of conservation agriculture affects topsoil aggregate-associated 15nitrogen but not the 15nitrogen use efficiency by wheat in a semi-arid climate

Available results on the evaluation of nitrogen (N) use efficiency in conservation agriculture (CA) are inconsistent and studies that cover all three components of CA are sparse, especially in tropical agro-ecosystems. Hence, CA effects on variations in total soil N (TSN) and aggregate-associated N-15 on a sandy loam soil were evaluated after N-15 urea (similar to 5% atom excess) application at 120 kg ha(-1) to wheat in the fourth year of an experiment in the Indo-Gangetic Plains (IGP). The cropping system during 2008-2011 was cotton (Gossypiwn hirsutum)-wheat (Triticum aestivum), that was changed to maize (Zea mays)-wheat in 2011-2012. Results revealed that plots under zero tillage with residue retention of both crops (ZT + R) had better topsoil (0-5 cm layer) aggregation and higher residual N-15 in soils than conventionally tilled (CT) plots. The ZT + R plots also had higher macro-aggregates (0.25-8 mm) with less silt + clay sized particles than the CT plots in the 5-15 cm soil layer. Furthermore, both tillage and residue management had significant effects on TSN and aggregate associated N stocks. The plots under ZT + R had similar to 9% higher TSN in topsoil than the CT plots and the ZT and ZT + R plots had about 48 and 34% higher large macro-aggregate associated N stocks than CT and CT + R plots, respectively. However, the potential N-mineralization data of the macro-aggregates of ZT + R (CA) and CT plots were similar. In addition, the fertilizer N-15 use efficiency, the residual fertilizer N-15 in soil (0-45 cm layer) and the calculated unaccounted fertilizer N-15 data of both CA and CT plots were also similar after the fourth year of cultivation. In summary, although CA (ZT + R) adoption had better soil aggregation and with more TSN accumulation within macro-aggregates, it could not affect the fertilizer N-15 use efficiency within a short period (4 years). Thus, with a short-term adoption of CA, fertilizer-N dose may not be reduced in a sandy loam soil of the IGP and similar agro-ecosystems.