# Periphyton closes the nitrogen budget gap in rice paddies

## TL;DR

Periphyton, a type of microbial community in rice paddies, helps explain the missing nitrogen in fertilizer budgets by temporarily storing and later releasing nitrogen.

## Contribution

The study identifies periphyton as a key, previously overlooked nitrogen sink in rice paddies using nationwide 15N tracing.

## Key findings

- Periphyton captures 6%–24% of applied nitrogen fertilizer in rice paddies.
- Sequestered nitrogen is stored as bioavailable ammonium and later released through decay.
- This pathway accounts for the missing nitrogen in previous agricultural nitrogen budgets.

## Abstract

Persistent 4%–22% gaps in nitrogen (N) mass balances have hindered sustainable N management in paddy agriculture. Periphyton are known N sinks, yet their role in paddies remains unclear. We used 15N tracing in 840 paddies across China to quantify periphyton-associated N pools and their fate. Periphyton captured 6%–24% (mean: 12%) of the applied N fertilizer (i.e. ∼0.8 Tg N yr−1 nationwide), effectively accounting for the missing N in previous budgets. Most of the sequestered N was stored as bioavailable ammonium. Partitioning analysis revealed that periphyton-mediated N was subsequently released into residual soil N (512–640 kt), denitrification (56–128 kt) and ammonia volatilization (64–232 kt). Thus, periphyton act as transient N reservoirs, immobilizing N fertilizer early in the growing season and gradually releasing it through biomass decay. This overlooked pathway closes a critical gap in agroecosystem N cycling and supports more precise N management in rice systems.

Periphyton, microbial aggregates at the soilwater interface, temporarily sequesters fertilizer nitrogen in rice paddies. This periphyton-mediated nitrogen cycling helps explain the long-standing `missing nitrogen' in fertilizer budgets and opens new opportunities to improve nitrogen-use efficiency.