# Effects of Hydrochar Incorporation on the Nitrogen Leaching Flux Pattern and Load in Rice Paddy Soil and Crop Production

## TL;DR

Adding hydrochar to rice paddies can boost crop yield and reduce nitrogen leaching, but the effects depend on the amount of hydrochar and nitrogen applied.

## Contribution

The study reveals how hydrochar application affects nitrogen leaching patterns and rice yield under varying nitrogen inputs.

## Key findings

- Rice yield was highest with 192 kg/ha nitrogen and lowest with 144 kg/ha.
- High hydrochar application increased ammonium leaching when nitrogen input was low.
- Organic nitrogen was the main leaching form, and hydrochar reduced it when nitrogen was reduced.

## Abstract

Hydrochar (HC) incorporation affects soil nitrogen (N) transformation, which could further affect the N leaching loss. We conducted a soil lysimeter experiment to evaluate the responses in terms of N leaching and rice yield to HC applied at a low (0.5%) or high (1.5%) rate, while considering three N inputs, i.e., 240, 192, and 144 kg/ha (named N240, N192, and N144, respectively). The results showed that the rice grain yield was highest (124.3 g/pot) for N192, while being significantly reduced to the minimum yield achieved in the study (110.3 g/pot) for N144. Interestingly, for the N input 144 kg/ha, HC application increased the rice grain yield by 6.9–8.0%, which was equivalent to that of N240. NH4+-N leaching occurred mainly during the first 4 weeks of the rice season, and HC did not influence NH4+-N leaching for both the N inputs, 192 and 240 kg/ha. However, compared to N144, N144 + HC1.5% recorded a significantly higher NH4+-N leaching loss of 34.6%. This suggests that the application of a high amount of HC increases the NH4+-N leaching risk when the N input is low. HC application resulted in 10.2–45.3% more NO3−-N leaching loss when the three N inputs were applied, the effect of which was significant in regard to the applications involving a 20 and 40% N reduction, but this occurred only with the applied treatments involving 1.5% HC. Moreover, we found that organic N was the main form of leachate N (>80%). More specifically, N144 + HC recorded 7.8–8.3% lower organic N leaching than N192. Based on the effects of HC on the rice grain yield and N leaching, we recommend applications involving a 40% N reduction (N144) with a lower amount of HC (HC 0.5%) to ensure high crop production and to protect the water environment.