Continuous application of biochar increases 15N fertilizer translocation into soil organic nitrogen and crop uptake in drip-irrigated cotton field

Purpose Biochar amendments rebuild the linkage between carbon (C) and nitrogen (N) cycles in the soil. The availability and transformation of N fertilizer are altered in the plant-soil system after biochar application. In this study, we investigated the effect of biochar addition on the transformation of N fertilizer into soil organic N (SON) fractions and cotton plant. Materials and methods A long-term field experiment was conducted from 2014 to 2019 to study the annual dynamic of soil total nitrogen (TN), cotton apparent nitrogen recovery (ANR), and yield after biochar application. Four treatments were set up as follows: (1) no N fertilizer and C amendment (CK), (2) N fertilizer (NF), (3) N fertilizer plus cotton straw (NF + CS), and (4) N fertilizer plus cotton straw biochar (NF + CB). A (15) N tracing micro-plot experiment was conducted based on the field experiment in 2019 to explore the long-term effect of biochar addition on the transformation of N fertilizer into SON fractions. Results and discussion NF + CS and NF + CB increased soil TN, cotton ANR, and yield from 2015 to 2019, and the NF + CB outperformed NF + CS from 2017 to 2019. Compared with NF and NF + CS, NF + CB significantly enhanced soil ammonia nitrogen (AN), amino sugar nitrogen (ASN), hydrolyzable unknown nitrogen (HUN), and non-hydrolyzable nitrogen (NHN) in 2019. (15) N micro-plot experiment indicated that NF + CB significantly increased cotton plant (15) N recovery (36.2% and 16.8%) and soil (TN)-N-15 recovery (41.7% and 10.2%) relative to NF and NF + CS, respectively. NF + CB improved the proportion of cotton N uptake derived from N fertilizer. Compared with NF, NF + CS increased (15) N recovery of labile SON fractions (amino acid nitrogen (AAN), AN, ASN) and the ratios of AA(15)N/(TN)-N-15 and AS(15)N/(TN)-N-15, but decreased the ratio of (NHN)-N-15/(TN)-N-15.On the other hand, NF + CB enhanced (15) N recovery of labile SON fractions and stable fraction (NHN) and promoted the transformation of more (15) N fertilizer into AAN, AN, ASN, and NHN. Conclusions Continuous application of N fertilizer with biochar increased nitrogen use efficiency and yield of cotton, promoted transformation of N fertilizer into soil organic N pool, and improved retention and stability of N fertilizer in drip-irrigated cotton field.