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).