4. Discussion
Decreasing topsoil thickness decreased the seed yield of maize but not soybean. The great reduction of root and shoot biomass at V7 was associated with the maize yield reduction under shallow topsoil depths compared to thick topsoil (Fig. 2), because biomass production at this growth stage was essential to maize yield formation (Russo et al., 2004; Herrmann et al., 2013). Similarly, Lin et al. (2019) found that there was no difference in soybean yield between “light” and “moderate” erosion sites, but 11% of yield loss was observed in maize grown in the “moderate” erosion sites. These results demonstrated that the increasing soil erosion reduced crop productivity and the reduction magnitude might be dependent of crop species (Bakker et al., 2007; Gao et al., 2015; Lin et al., 2019).
The decreases in seed yield and biomass with the decrease of topsoil thickness might result from the reduction of nutrient supply in soil (Sanaullah et al., 2012; Bista et al., 2018). Our present study showed that 10 cm of topsoil depth decreased the N, P and K uptake per unit root length of both maize and soybean expect maize at R3 (Fig. 4). As N, P and K are considered as the most important major nutrients for crop physiological function and production capacity (Wang et al., 2017), this study showed the linear correlations of total and specific nutrient uptake with total shoot biomass production (data not shown).
As soil nutrients are generally mainly distributed in the topsoil (Liu et al., 2013; Zhao et al., 2016), the decrease of topsoil thickness also decreased root nutrient acquisition. It was evident that nutrient uptake per unit root length of both crops was lower in 10 cm than 30 cm of topsoil depth at both vegetative and reproductive stages. Zöbisch et al. (1995) reported that 3−6 t ha-1 loss of topsoil resulted in 24% and 28% of decreases in P and K uptake in maize, and 15% and 16% in beans. In addition, soil erosion reduced soil ammonium concentration by 66% at the Loess Plateau (Zheng et al., 2005), and soil organic C, total N and total P by 33%, 28% and 27%, respectively, at an alpine grassland (Nie et al., 2013). In this study, the decrease of the topsoil depth resulted in the substantial decreases in plant N and P uptake, and decreases in the available N and Olsen P concentrations in rhizosphere soil as well, but not much in available K. Our results indicated that the impact of the soil erosion on N and P cycles would constrain the crop productivity (Fig. 5). The stimulation of the urease, phosphatase and invertase activities in the rhizosphere of maize (Fig. 6) also indicated the scares of N and P availability to plants drove the biochemical processes of the N and P mineralization. In comparison, soybean yield did not respond to the decrease of topsoil thickness in this study. Similarly, Sui et al. (2009) found that soybean yield did not decrease by the removal of 5 cm topsoil, which had decreased maize yield by 10% in a black soil. It appeared that maize was more sensitive to topsoil erosion than soybean, which could be attributed to various capabilities of nutrient acquisition between crops. Nitrogen uptake per unit root length of maize was dramatically decreased at V7 than soybean at R1 under 10 cm of topsoil depth in comparison to 30 cm. This also implied that soybean plants were capable of fixing N2 particularly when soil N fertility decreased due to the erosion/removal of fertile topsoil. In terms of P uptake, P availability under 10 cm of topsoil might not meet the nutrient demand of maize as low availability of P in the rhizosphere of maize and stronger phosphatase activity which can mineralize soil organic P for maize plants (Fig. 6). However, it did not occur in the rhizosphere of soybean. Some studies had found that soybean not only alleviated N deficiency due to N2-fixation (Li et al., 2017), but also had less P limitation than non-leguminous crops which had resulted from rhizosphere acidification during N2 fixation to mobilize the non-labile P (Li et al., 2019b). Although low concentrations of available nutrients could stimulate enzymatic catalysis, mineralized nutrients might not sufficiently mitigate nutrient constraints to maize under erosion (Berhe et al., 2012). Since the quantity of enzymes is largely determined by microbial activity and the content of soil organic matter (Wang et al., 2012; Nie et al., 2015), further study is needed to understand how the soil microorganisms mineralize soil organic matter under topsoil erosion, considering C input and soil nutrient supply in the maize-soybean rotation system.5. ConclusionIncreasing topsoil erosion decreased yield of maize more severely than soybean. The deficiencies of N and P in maize under soil erosion might greatly constrain the productivity even though the nutrient mineralization was stimulated. Soybean was more resilient to topsoil erosion than maize. Increasing N and P supply would be applicable to alleviate yield loss of maize grown in eroded soils.Acknowledgement: This research was partially funded by the National Key R&D Program of China [2017YFC0504200; 2017YFD0300300], the National Natural Science Foundation of China [41671274], and partially supported by the Professional Association of the Alliance of International Science Organizations (grant number ANSO-PA-2020-12).