Long-term green manuring enhances crop N uptake and reduces N losses in rice production system

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Highlights

  • Long-term green manure substitution significantly improves soil mineral N.

  • Long-term green manure substitution enhances crop N uptake and reduces N losses.

  • The WHCNS_LIXIM model performed well in simulating soil mineral N and crop N uptake.

  • Milk vetch application of 15–30 t ha−1 was recommended to improve crop N uptake and reduce N losses.

Abstract

Green manuring is a potential method for reducing chemical N fertilizer input and reactive N losses to the environment that can achieve dual goals of food security and green development of agriculture. In this study, three long-term field experiments in south China and process-based model (WHCNS_LIXIM model) were combined to examine effects of long-term green manure substitution on soil N supply, crop N uptake, and N losses in rice production system. Results showed that green manure substitution significantly improves soil mineral N by 27.4–94.9% and 1.0–45.0% at jointing and maturity stages, respectively, compared with the use of chemical N alone. Moreover, 18.4–62.2% of crop N uptake increased at the maturity stage at Xinyang and Yujiang sites. The model demonstrated satisfactory performance in modeling dynamics of soil mineral N, crop N uptake, crop dry matter, and grain yield of the rice–green manure system with a determination coefficient (R2) > 0.967 and slope of linear regression equation (β) close to 1.0. Simulated N budgets presented that green manure reduces 20–40% of the chemical N fertilizer input and decreased 5.5%, 25.1%, and 30.6% of N losses by ammonia volatilization, leaching, and runoff, respectively, when green manure application rates (fresh matter) are less than 30 t ha−1. However, the total N loss increased with the increase of green manure application amounts when application amounts are larger than 30 t ha−1. These findings indicated that the further reduction of chemical N fertilizer was needed when the application amount of milk vetch was higher than 30 t ha−1. A milk vetch application of 15–30 t ha−1 was recommended to substitute 20–40% of chemical N fertilizer to improve crop N uptake and reduce N losses, and develop green rice production system in the study areas.

Introduction

Green manuring is a traditional cultivation practice that has played an essential role in the improvement of soil fertility and crop yield in the long history of agricultural production in China (Cao et al., 2017). Green manure has attracted considerable research attention in the 21st century because it can achieve dual goals of food security and green development of agriculture. Several field experiments were recently conducted to examine effects of green manuring on crop growth and N losses (Xie et al., 2016, Mohanty et al., 2020). The results showed that green manuring is a promising method for reducing chemical N fertilizer input and reactive N losses in the environment while maintaining crop yield (Gao et al., 2020, Fontaine et al., 2020).

Milk vetch (Astragalus sinicus L.) is a typical leguminous green manure used in the rice system, grown during fallow season, and incorporated into the soil at their full-bloom stage before transplanting rice. The fixed atmospheric N is released into the soil as the mineral N form with the decomposition of milk vetch. The enhancement of the soil N supply via milk vetch incorporation can satisfy the N requirement of the subsequent rice. Based on 11 field experiments in south China, researchers found that milk vetch can replace 20–40% of chemical N fertilizer while slightly increasing rice yield compared with conventional chemical N management practices (Gao et al., 2020). The cultivation and utilization of milk vetch showed high potential in reducing the chemical N input and N losses while ensuring rice yield (Zhao et al., 2015, Yao et al., 2017). Previous studies demonstrated that green manuring can substantially reduce N losses through leaching (Zhang et al., 2016), runoff (Zhao et al., 2013), ammonia volatilization (Bai et al., 2015), and N2O emission (Mohanty et al., 2020). However, long-term studies are needed to ensure the effectiveness of milk vetch incorporation as an alternative approach for reducing chemical N fertilizer input and N losses while maintaining rice yield (Lee et al., 2010).

Quantifying N mineralization from green manure is a key process in evaluating crop N use efficiency and developing optimal management practices for rice–green manure systems. However, direct in situ measurement of N mineralization is labor-intensive, time-consuming, and unreliable. Process-based models provide a powerful method for quantifying effects of various field management practices on N fates and crop growth (Mary et al., 1999, Hansen et al., 2012, Liang et al., 2019). Constantin et al. (2011) used the LIXIM model to quantify cumulative impacts of green manure incorporation on net N mineralization and N fates; the results showed that green manuring enhances soil N mineralization (9–26 kg N ha−1 yr−1) and crop N uptake. Clivot et al. (2017) successfully quantified in situ net N mineralization from soil organic matter in 65 arable bare soils in France using the same model. Yin et al. (2020) calculated the soil N mineralization rate and N fates in a 34-year crop rotation field and reported that the model captures long-term effects of green manuring on N mineralization and N fates. The LIXIM model was recently integrated into the soil water heat carbon nitrogen simulator (WHCNS) model to improve the simulation of soil net N mineralization and N fates of the rice system, and the integrated model (WHCNS_LIXIM) can be used to evaluate water and N use efficiencies of different rice production systems in China (Liang et al., 2019). The integrated model showed a good performance in modeling N fates under different N management practices for rice system (Shi et al., 2020, Liang et al., 2021). Therefore, the WHCNS_LIXIM model was adopted in this study to estimate soil net N mineralization and N fates of the rice–green manure system.

This study aims to (i) examine effects of green manure substitution on soil N supply and crop growth on the basis of three long-term field experiments; (ii) test the performance of the WHCNS_LIXIM model in simulating soil N dynamics, crop N uptake, and crop growth for the rice–green manure system; and (iii) evaluate effects of green manure substitution on N mineralization and N losses.

Section snippets

Site description

The three field experiment sites are located in a major rice production area of south China (Table 1). Xinyang site (32.13° N, 114.08° E) in Henan Province belongs to the subtropical–warm transitional zone, and Yujiang (28.20° N, 116.81° E) and Gaoan (28.25° N, 115.12° E) sites in Jiangxi Provance are in the humid subtropical monsoon climate zone. Annual mean temperature (or precipitation) is 16.6 °C (1776 mm), 17.6 °C (1789 mm), and 17.7 °C (1560 mm) for Xinyang, Yujiang, and Gaoan sites,

Crop dry matter and grain yield

Crop dry matter (DM) at different stages of rice growth is presented in Table 5. Differences in DM among various treatments at all sites were negligible, and the average DM at maturity was 19,606, 13,881, and 10,256 kg ha−1 for Xinyang, Yujiang, and Gaoan sites, respectively (Table 5). Crop yield of green manure-incorporated treatments demonstrated no significant differences with the F100 treatment at Xinyang and Gaoan sites. The incorporation of green manure had no significant impact on DM but

Effects of green manure incorporation on crop growth and N uptake

Gao et al. (2020) used datasets from 11 field experiments in south China and revealed that the incorporation of milk vetch saves 20–40% of chemical N fertilizer and slightly increases crop yield compared with conventional chemical N management practices. Lee et al. (2010) evaluated effects of different application rates of milk vetch on rice yield and crop N uptake and showed that the significant increase of rice yield from the increase of milk vetch application from 0 t ha−1 to 20 t ha−1 is

Conclusion

The results of three long-term field experiments showed that green manure-applied treatments enhance SMN from 27.4% to 94.9% at the jointing stage and from 1.0% to 45.0% at the maturity stage compared with the use of urea alone. The crop N uptake increased with the increase of green manure application rates, and the average crop N uptake increased from 147.8 kg N ha−1 to 254.6 kg N ha−1 when the green manure application rate increased from 0 t ha−1 to 45 t ha−1. The WHCNS_LIXIM model

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The study was funded by the National Key Research and Development Program of China (2021YFD1700200), the National Natural Science Foundation of China (No. 42007071), the Earmarked Fund for Modern Agro-industry Technology Research System-Green Manure (CARS-22).

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