Nitrogen levels regulate intercropping-related mitigation of potential nitrate leaching
Graphical Abstract
Introduction
Intensive agricultural production for high output has resulted in a myriad of environmental impacts in rapidly developing countries, especially in China and India (Carlson et al., 2017, Li et al., 2016). Coincidentally, intercropping as a common practice of diversified cropping was reported to increase yields by 1.7–2.1 Mg ha-1 and gross income by 33% relative to monocropping (Li et al., 2020, Martin-Guay et al., 2018). In terms of potential environmental benefits, intercropping is expected to play a crucial role in addressing agricultural production harmony between yield and environmental impact in these regions (Tilman, 2020).
Nitrate-N (NO3−-N) leaching from farmland is a primary source of water contamination by N (Sebilo et al., 2013). NO3−-N leaching losses are strongly associated with NO3−-N accumulation in the soil profile, especially below the root layer, which represents the potential capacity of NO3−-N leaching (Lu et al., 2019). Previous studies have confirmed that NO3−-N leaching is substantially lower under intercropping than monocropping (Kintl et al., 2018, Nie et al., 2012, Pappa et al., 2011). These studies identified the cropping patterns that are more advantageous in reducing NO3−-N leaching. However, there were discrepancies in N input, crop distribution, and yield output between intercropping and monocropping; therefore, it is difficult to compare and distinguish the effects of intercropping (interspecific interaction) on NO3−-N leaching when the crop composition and N input differ between cropping patterns (Xie and Kristensen, 2016a). More importantly, the mechanism underlying intercropping-related NO3−-N leaching has not been adequately explored (Tian et al., 2018).
Soil NO3−-N leaching is controlled by the NO3−-N content in the root layer and downward water migration (Huang et al., 2018). In addition to N uptake by crops, there are various factors that affect the dynamics of soil NO3−-N, including nitrification and denitrification. Nitrification potential (NP) can reflect the capacity of soil nitrification, which usually increases following N application (Ouyang et al., 2016). NO3−-N is transformed from NH4+-N fertilizer or mineralized from organic N, mainly through aerobic chemoautotrophic nitrification by nitrifying prokaryotes (Islam et al., 2007). Ammoxidation is the rate-limiting step in chemoautotrophic nitrification, which is catalyzed by ammonia monooxygenase (AMO) (Norton et al., 2002). Both ammonia-oxidizing bacteria (AOB) and archaea (AOA) produce AMO. The α-subunit of AMO is encoded by amoA, which has been selected as a molecular marker to explore the abundance of ammonia oxidizers (Leininger et al., 2006). NO3−-N, contrarily, is reduced to NO2−-N and NOx or N2 through denitrification, and the initiation of NO3−-N reduction is catalyzed by membrane-bound or periplasmic nitrate reductase encoded by narG or napA (Zumft, 1997).
Intercropping has been shown to alter soil and crop factors regulating soil nitrification and denitrification, including temperature, moisture, labile carbon, and nutrient supply of soil (Amossé et al., 2014). The AOA abundance in rhizospheric soil increased 1.42 times under maize intercropped with faba bean compared to monocropping, but there was no significant difference in AOB between intercropping and monocropping (Zhang et al., 2015). However, little is known about how intercropping affects the NO3−-N transformation intensity and the number of functional microorganisms in field non-rhizospheric soil (Zhao et al., 2017). Furthermore, these effects have not been utilized to explore the underlying mechanisms regulating NO3−-N leaching in intercropping (Niklaus et al., 2006).
Maize–potato intercropping, as a typical diversified cropping system of high- and low-stalk crops, has been extensively adopted in Asia, Africa, and Latin America (Mushagalusa et al., 2008). Compared with monocropping, maize–potato intercropping had significant advantages in yield and plant N uptake, but they were affected by the N application rate (Singh et al., 2015). We hypothesized that maize–potato intercropping could enhance crop N uptake, accelerate the transformation of soil NO3−-N, and effectively reduce potential NO3−-N leaching, and these effects were regulated by N application rate. This study focused on (1) the effect of intercropping on NO3−-N accumulation below the root layer and its regulation via N application rate compared with monocropping; (2) the response of soil NP and the abundance of NO3−-N transformation-related microorganisms to intercropping and N application rates, and (3) the critical drivers influencing NO3−-N accumulation below the root layer under intercropping.
Section snippets
Site description
A multi-year field plot experiment was established in March 2014 at the Daheqiao Experimental Base (23°32ˊ N, 103°13ˊ E, 1953 m above sea level) in Kunming, Yunnan Province, China. With a subtropical monsoon climate, this site has 2077.7 h of annual sunshine and a 257 d frost-free period. The annual precipitation is 1040 mm, approximately 70% of which occurs from June to September. The soil type in this region is called red soil (Ferralic Cambisol, FAO, 2006). Prior to the experiment
NO3−-N accumulation in soil profile
Soil NO3−-N accumulation gradually declines within the root layer (0–45 cm), but varies slightly below the root layer (45–75 cm) with increasing depth (Fig. 2). The NO3−-N accumulation increased with N application rate in the soil profile, especially in the root layer. Between cropping patterns, NO3−-N accumulation in the root layer varied greatly. However, NO3−-N accumulation below the root layer, that is, potential NO3−-N leaching, is invariably lower in intercropping than the weighted mean
Mitigating effect of intercropping on potential nitrate leaching and N regulating mechanism
We used the NO3−-N accumulation in the 45–75 cm soil layer to characterize potential NO3−-N leaching. Referring to previous studies (Qin et al., 2004; Chen et al., 2017), it is assumed that maize roots are distributed within 45 cm. A new study next to this experiment site showed that maize roots were distributed within 40 cm, when the same variety of maize was grown on the same red clay soil (data not yet published). Contrarily, 75 cm was used as the lower limit to analyze the potential NO3−-N
Conclusion
The area-scaled soil NO3−-N accumulation below the root layer was mitigated by 15.8% under maize and potato intercropping compared with the weighted mean of maize and potato monocropping. Intercropping-related mitigation of NO3−-N accumulation increased with N application rate. Both intercropping and N application enhanced soil NP. The amount of AOA increased under intercropping, but the responses of AOB amoA and narG to intercropping were small. Improved N uptake by maize was a critical
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.
Acknowledgement
This project was supported through grants from the National Science Foundation of China (41967004, 41201289, and 41361065), the Agricultural Joint Project of Yunnan Province (2017FG001-027), Young and Middle-aged Reserve Talents Project of Yunnan Province (2017HB027) and the Key Research and Development Program of Yunnan Province (2018BB015). We acknowledge with appreciation the assistance of Long Zhou, Qilin Zhu, Xinling Ma, Xiaomin Qin, Shengli Zhao, Yu Lv and Hanling Qian in field and
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