Simultaneously improving yield and nitrogen use efficiency in a double rice cropping system in China

Highlights

• Rice yield needs to increase for food security.

• The Nutrient Expert (NE) improved rice yields and nitrogen uptake compared with farmers' practice.

• NE enabled the nutrients in rice vegetative organs to be more efficiently translocated.

• NE is suitable as a nutrient strategy for the double rice system.

Abstract

The Nutrient Expert system (NE) has been proposed to improve yield and nitrogen (N) use efficiency (NUE) in the double rice cropping systems in China. However, the advantage of the NE system has yet to be quantified experimentally. A four-year field experiment was conducted in a double rice cropping system in China, to evaluate the ability of NE in improving yield and NUE. The experimental treatments consisted of NE based fertilization, current farmers’ practices (FP) and soil test based fertilizer recommendation (ST), and a series of nitrogen (N) rate treatments. The NUE decreased with increasing N application, while the yield did not increase significantly beyond N application rates of about 140 kg ha⁻¹ (corresponding to the amount proposed by NE) in both early and late rice. NE increased grain yield (by 10.3% and 6.3%) and N uptake (by 5.7% and 4.0%) compared with FP and ST, respectively. NE significantly increased NUE compared with FP, and decreased the N surplus in comparison to FP and ST. The N dilution curve was Nc = 34.50 W^−0.55 for early rice and Nc = 37.71 W^−0.59 for late rice (where Nc is the N concentration in g kg⁻¹, and W is the dry matter accumulation in t ha⁻¹). The relationship between relative yield and the nitrogen nutrition index derived from the dilution curves confirmed that NE offered an optimum N application rate (approximately 140 kg ha⁻¹) for both early and late rice. Carbon (C) and N translocation from vegetative organs to grains was enhanced with increasing N rate, while NE significantly increased C and N translocation compared with FP. Overall, the NE system ensured a high rice yield, increased N uptake and NUE. Therefore, the NE, as a user-friendly tool, is a sustainable fertilizer recommendation approach suitable for double rice cropping system, especially when soil testing is not available or timely for smallholders.

Introduction

Nitrogen (N) is considered to play a crucial role in high-yielding production of rice (Oryza sativa L.) (Kiba and Krapp, 2016). In recent decades, the rice yield has increased significantly in China; this is primarily attributed to the genetic gain and increase in input of inorganic fertilizers, especially N fertilizers (Ju et al., 2009, Miao et al., 2010, Li et al., 2019). However, applying N fertilizer should aim to obtain both higher yields and better N use efficiency (NUE) (Mueller et al., 2012). A smart N supply can provide a high NUE and considerable economic benefits (Pan et al., 2012). Farmers, however, apply N in hope for high productivities, and often tend to over-fertilize, leading to low NUE of rice production in China (Vitousek et al., 2009, Peng et al., 2010). Over-fertilization is detrimental to plant development, reproduction, and grain quality (Mikkelsen and Hartz, 2008). Moreover, over-fertilization causes a series of environmental issues, e.g., greenhouse gas emissions (Kahrl et al., 2010, Liu et al., 2015), soil fertility degradation (Reidsma et al., 2012), and ground and surface water pollution (Letey and Vaughan, 2013).

To increase grain yield without creating environmental problems, fertilizer recommendation based on soil testing (ST) has been implemented in China since 2005, and promoting this activity at a large scale has yielded positive results (He et al., 2009, Yusuf et al., 2009). Although this activity has reduced the inappropriate use of fertilizers, the correlations between soil testing values and crop yields are very low especially for rice (Dobermann et al., 1995). Moreover, soil testing entails a cumbersome process, takes a long time and is expensive, and is therefore not accessible to smallholder farmers (Tang et al., 2021). A site-specific nutrient management method based on yield response and agronomic efficiency was jointly developed by the Institute of Agricultural Resource and Regional Planning, Chinese Academy of Agricultural Sciences, China (IARRP, CAAS) and the International Plant Nutrition Institute (IPNI) to surmount above limitations. These institutes developed the so-called Nutrient Expert (NE) system for rice (Xu et al., 2017).

This NE system uses computer-based decision support technology and a questionnaire to provide a simple advice despite the complexity of the principles behind fertilization (He et al., 2022, Xu et al., 2017). The NE system aims to provide 4 R nutrient stewardship (using fertilizers with the Right source, at the Right rate and at the Right time, and in the Right place) based on a field management method for farmer fields. The NE system has been widely used in rice production, especially in terms of right fertilizer sources and fertilization places that are supported by the local agricultural extension system. Yet, the ability of the NE system in improving rice grain yield and NUE needs experimental confirmation. Moreover, there are still no reports on the physiological bases of increasing grain yield and NUE by NE. Here, we hypothesize that the NE system promotes the translocation of nutrients from source organs to sink organs during grain filling, thus increasing rice grain yield and NUE.

It is well-known that the analysis of NUE of any fertilization strategies such as NE or ST must refer to the crop N demand and then assess if crop N demand is fully satisfied by N supply. The more this N demand is satisfied, the less will be the response of the crop to N fertilization. There is a well-established method for estimating the extent of crop N demand satisfaction: the Nitrogen Nutrition Index, NNI (Lemaire et al., 2008, Lemaire and Ciampitti, 2020). The NNI values can be derived from the known fact that the N concentration (Nc) in a crop decreases with an increase in shoot biomass (Greenwood et al., 1990). This decrease in N concentration can be expressed as a power function called the “critical N-dilution curve” (Lemaire et al., 2008), i.e, the curve for the minimal concentration of total N in shoots that produced the maximum aerial dry matter. The N dilution curve helps dynamically diagnosing the N status in the crop’s vegetative stages, which is crucial to evaluate plant N demand, predict crop yield and optimize N management. The NNI can be obtained by dividing the actual plant N concentration by the Nc value determined by the N dilution curve, and NNI can be used as a practical diagnostic tool for analyzing N status in plant (Lemaire and Meynard, 1997, Ziadi et al., 2008). So far, there is a lack of data on the evaluation of NUE using the N dilution curve and the NNI to assist in evaluating the fertilizer management performance of the NE system.

The objectives of this study are: (i) to demonstrate the ability of the NE system in improving rice grain yield and NUE, (ii) to use the “critical dilution curve-NNI” framework to confirm the validity of the NE system, and (iii) to analyze the crop physiological basis of the simultaneous improvement of yield and NUE. We do so by determining grain yield, N uptake dynamics, NUE and nutrient translocation of rice under various nutrient management scenarios and N application rates, based on a four-year experiment with early and late rice planting, thus encompassing eight growing seasons in China.