Divergent gross nitrogen transformation paths in the topsoil and subsoil between abandoned and agricultural cultivation land in irrigated areas

The nitrate concentration in groundwater has increased in many irrigated areas worldwide due to the excessive use of both water and fertilizers. Abandoned farmlands in such irrigated areas may alter the nitrogen (N) cycle because of drastically changed water and N inputs. However, the mechanisms of the N cycle in response to such changes remain unclear. We studied biogeochemical N cycling and microbiological responses from abandoned arable lands (AF), for the topsoil (20 cm depth) and subsoil (100 cm depth) layers, in comparison with irrigation-fertilization (control = CK) land, by using 15N tracing techniques, the 16S rRNA gene, and real-time PCR (qPCR) to reveal the mechanisms underpinning the N cycle. We found that the biogeochemical environment of abandoned soils shifted their N-cycling pathways. Except for reduced soil moisture, soil properties of total C and N, as well pH, showed improvement in the two layers of AF. But the microbial abundances of ammonia-oxidizing bacteria (AOB-amoA), archaea (AOA-amoA), bacteria and fungi were all significantly lower in the AF; and they presented a consistent trend in the subsoil of the two lands. Significant differences in gross N transformation rates were found for mineralization rates (MN) and autotrophic nitrification rate (ONH4) between lands or depths. Compared with AF, MN was increased by 1.45- and 11.75-times, and ONH4 by 1.69- and 2.89-times in the topsoil and subsoil of CK, respectively. Our results suggest that the SM × C/N interaction provides insight into the mechanisms underlying the soil microbe-driven changes to transformation rates in nitrogen dynamics after abandoning water-limited lands. The high moisture and N inputs reported here highlight the dynamics and prevalence of MN and ONH4, and an increasing the nitrate leaching rate in the unsaturated zone, which poses a major threat to groundwater quality. Long-term abandonment of farmland soils (AF) in an irrigated area reduced the gross N mineralization and nitrification rates, and the latter changed markedly with soil depth of the two lands. The graphical abstract highlights a hierarchy of controlling variables in operation, of which soil moisture is a powerful factor that co-varies for the sustained N supply and other biogeochemical variables driving N mineralization and nitrification. [Display omitted] •SM × C/N interaction improved microbial abundance but not their diversity in lands and depths.•The AOB-amoA in CK was 44–143 times greater than