Tea plantation destroys soil retention of NO3− and increases N2O emissions in subtropical China

The intensive conversion from woodland to tea plantation in subtropical China might significantly change the potential supply processes and cycling of inorganic Nitrogen (N). However, few studies have been conducted to investigate the internal N transformations involved in the production and consumption of inorganic N and N2O emissions in subtropical soils under tea plantations. In a 15N tracing experiment, nine tea fields with different plantation ages (1-y, 5-y and 30-y) and three adjacent woodlands were sampled to investigate changes in soil gross N transformation rates in humid subtropical China. Conversion of woodland to tea plantation significantly altered soil gross N transformation rates. The mineralization rate (MNorg) was much lower in soils under tea plantation (0.53–0.75 mg N kg−1 d−1) than in soil sampled from woodland (1.71 mg N kg−1 d−1), while the biological inorganic N supply (INS), defined as the sum of organic N mineralized into NH4+ (MNorg) and heterotrophic nitrification (ONrec), was not significantly different between soils under woodland and tea plantation, apart from soil under 30-y tea plantation which had the largest INS. Interestingly, the contribution of ONrec to INS increased from 19.6% in soil under woodland to 65.0–82.4% in tea-planted soils, suggesting ONrec is the dominant process producing inorganic N in tea-planted soils. Meanwhile, the conversion from woodland to tea plantation destroyed soil NO3− retention by increasing ONrec, autotrophic nitrification (ONH4) and abiotic release of stored NO3− while decreasing microbial NO3− immobilization (INO3), resulting in greater NO3− production in soil. In addition, long-term tea plantation significantly enhanced the potential release of N2O. Soil C/N was positively correlated with MNorg and INO3, suggesting that an increase in soil C/N from added organic materials (e.g. rice hull) is likely to reduce the increased production of NO3− in the soils under tea plantation.