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

Xiuhua Liu; Chaochao Guo; Shuaishuai He; Hongyan Zhu; Junyuan Li; Zhaoyu Yu; Yan Qi; Junqi He; Jinbo Zhang; Christoph Müller

doi:10.1016/j.scitotenv.2020.137148

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

Sci Total Environ. 2020 May 10:716:137148. doi: 10.1016/j.scitotenv.2020.137148. Epub 2020 Feb 5.

Authors

Xiuhua Liu¹, Chaochao Guo², Shuaishuai He², Hongyan Zhu³, Junyuan Li², Zhaoyu Yu², Yan Qi², Junqi He², Jinbo Zhang⁴, Christoph Müller⁵

Affiliations

¹ Shaanxi Key Laboratory of Land Consolidation, Chang'an University, Xi'an 710054, China; Key Laboratory of Subsurface Hydrology and Ecological Effect in Arid Region of Ministry of Education, Chang'an University, Xi'an 710054, China; School of Water and Environment, Chang'an University, Xi'an 710054, China. Electronic address: Liuxh68@chd.edu.cn.
² Shaanxi Key Laboratory of Land Consolidation, Chang'an University, Xi'an 710054, China; Key Laboratory of Subsurface Hydrology and Ecological Effect in Arid Region of Ministry of Education, Chang'an University, Xi'an 710054, China.
³ Institute of Water Resources and Hydro-electric Engineering, Xi'an University of Technology, China.
⁴ School of Geography Sciences, Nanjing Normal University, Nanjing 210097, China.
⁵ Institute of Plant Ecology, Justus-Liebig University Giessen, Heinrich-Buff-Ring 26, 35392 Giessen, Germany; School of Biology and Environmental Science, Earth Institute, University College Dublin, Belfield, Dublin, Ireland.

PMID: 32059300
DOI: 10.1016/j.scitotenv.2020.137148

Abstract

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 (M_N) and autotrophic nitrification rate (O_NH4) between lands or depths. Compared with AF, M_N was increased by 1.45- and 11.75-times, and O_NH4 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 M_N and O_NH4, and an increasing the nitrate leaching rate in the unsaturated zone, which poses a major threat to groundwater quality.

Keywords: Abandoned farmland; Gross N transformations; Irrigation; Microbial abundance; Soil moisture; Topsoil and subsoil.