Straw returning mediates soil microbial biomass carbon and phosphorus turnover to enhance soil phosphorus availability in a rice-oilseed rape rotation with different soil phosphorus levels
Introduction
Phosphorus (P) is one of the nutrients limiting crop production; 30–40% of the world’s cropland is deficient in P (Atere et al., 2019, Holford, 1997, Vitousek et al., 2010). Consequently, large amounts of chemical P fertilizers are applied in agricultural production (Tilman et al., 2002). However, the P utilization efficiency is usually low due to the strong ability of organic matter and other minerals in the soil to fix P (Li et al., 2011, Zhu et al., 2018, Yan et al., 2013, Guppy et al., 2005). Excessive P in soils may increase its loss and aggravate potential environmental risks (Robertson and Nash, 2008, Xu et al., 2019). Therefore, improving soil legacy P availability and P utilization efficiency could provide conditions for reducing chemical P fertilizer inputs. P cycling through microbial biomass often determines the turnover and bioavailability of P (Boitt et al., 2018, Richardson et al., 2009, Schneider et al., 2017). In low-P soil, soil microbes can immobilize P rapidly and the demand for P will consume large amounts of organic C to drive Po mineralization (Bünemann et al., 2012, Romanyà et al., 2017). In contrast, in high-P soil, microbially dominated P cycling may be weakened, which requires less organic C (Pistocchi et al., 2018). Therefore, the differences in P cycling by microorganisms in soils with different P levels may be affected by the availability of C. In addition, changes in conditions such as soil moisture and temperature could affect microbial activity and C availability, and thus affect C stimulating P bio-activation (Huang et al., 2017b; Keiluweit et al., 2017; Zhou et al., 2014). The microbial mechanisms of P immobilization or mobilization are critical to reducing the risk of P leaching in soils. Soil MBP turnover can both release Pi and participate in the formation of Po (Schneider et al., 2017, Liang et al., 2020). Therefore, the turnover of P within the microbial biomass itself may change the composition of P in the soil. The P released from the soil MBP pool occurs as orthophosphate and organic forms, which is highly effective for plants (Hofmann et al., 2016, Schneider et al., 2017). Microbes motivate P for crop absorption through their biomass turnover and mineralization, which requires labile C for support (Hofmann et al., 2016, Spohn and Kuzyakov, 2013, Wang et al., 2016a). Consequently, organic C addition is likely to increase soil P utilization. Previous studies have shown that glucose or cellulose addition could stimulate microbial multiplication, thus extending the hotspots of acid and alkaline phosphatase to promote P mineralization (Jing et al., 2017, Wei et al., 2019). Moreover, the application of organic C shifted the soil microbial stoichiometry and associated microbial populations, thereby affecting the soil P cycle (Bi et al., 2020, Garbuz et al., 2021, Heuck et al., 2015). To data, most studies have focused on the relationship between labile C input and microbial community structure and P activity in soils. However, the effect of MBP turnover on the distribution of soil Pi and Po forms has rarely been reported.
In general, MBP cycles are affected by the C demand of the microbial community (Spohn and Kuzyakov, 2013, Wang et al., 2016a). Therefore, labile C input is critical for MBP turnover. Due to the pressures of population and cultivated land resources, developing countries, including China, produce large amounts of straw because they employ two or more crop rotations a year (Yin et al., 2018). Straw returning is the simplest and most effective method of agricultural C input (Liu et al., 2014). While the effect of straw C on the transformation responses of soil nitrogen has received considerable attention in the past (Wang et al., 2018), relatively few studies have investigated the effect of straw C on P turnover. Unlike the addition of a single carbon source, straw C is continuously released into the soil in natural ecosystems with mineral nutrients (Guan et al., 2020). In addition, straw returning can promote microbial activity, enhance soil native organic matter decomposition, increase the competition between low molecular weight compounds and phosphate, and change available P fractions (Fang et al., 2018, Guppy et al., 2005). Our previous research found that straw returning enhances the soil Olsen-P content (Wang et al., 2020). However, the response mechanism of soil P turnover to straw returning is still unclear.
Paddy-upland rotation is the most representative and widely distributed farming system worldwide and plays an essential role in ensuring global food security. This system could improve the adverse effects in the long-term-flooded paddy fields, change the form and availability of soil nutrient elements, and lead to a greater diversity of microbial communities (Zhou et al., 2014). P fertilization is an important measure to ensure crop yield in paddy-upland rotations. This system requires the excavation of soil P supply potential and the reduction of P fertilizer input to decrease environmental risks. Furthermore, the soil C and P cycles have more complicated changes with seasons because paddy-upland rotation fields frequently cycle between wetting and drying under anaerobic and aerobic conditions (Keiluweit et al., 2017, Zhou et al., 2014). It has been shown that frequent application of straw can increase the accumulation of organic C (Xu et al., 2011, Liu et al., 2014), which usually enlarges the soil MBP pool. Some studies have observed an increase in MBP turnover in the case of plant biomass restoration or organic fertilizer input (Boitt et al., 2018, Liebisch et al., 2014). The above studies focused on upland soils. However, there are seasonal changes in soil moisture and temperature in the paddy-upland rotation. Thus, the changes in the soil MBP pool and the MBC:MBP ratio in different rotation seasons under annual P application and straw returning and the effect of MBP turnover on P fractions need to be further clarified.
In this study, a 2-year rice (Oryza sativa L)-oilseed rape (Brassica napus L) rotation was evaluated in two field experiments with different soil Po levels. The aims of this research were (i) to assess the soil P supply capacity and seasonal difference in rice-oilseed rape rotation under P application; (ii) to survey the effects of P fertilizer with straw addition on the soil MBP pool, MBC:MBP ratio and its seasonal shifts; and (iii) to estimate MBP turnover under P fertilizer and straw addition and its effect on the distribution of soil Pi and Po forms. The insight obtained from this study will help to understand the internal mechanisms by which straw returning improves soil P availability and crop P utilization by accelerating MBP turnover.
Section snippets
Site description
Field experiments with different soil Po levels were performed in the cities of Wuxue (WX, 30°06′33″N 115°35′15″E) and Wuhan (WH, 30°28′10″N 114°21′21″E) in Hubei Province, China. The land use type at the WX and WH sites was arable land before beginning the experiment. Two field trials were started in June 2014 and October 2014. This study was conducted from the 2018 rice season to the 2020 oilseed rape season. The climate was a subtropical monsoon climate, with average temperatures of 17.1 ℃
Crop yields, P uptake and utilization efficiency
In the 2-year rice-oilseed rape rotations (Table 2, Table 3), P fertilization had higher effects on crop yield and shoot P uptake in Po-low soil (WH: 22.4%−850.2% and 79.1%−1356.3%, respectively) than in Po-high soil (WX: 5.3%−42.5% and 15.4%−104.5%, respectively). The increased rate of P application in the oilseed rape yield and the P uptake were higher (41.6%−850.2% and 84.8%−1356.3%, respectively) than in rice (5.3%−142.3% and 15.4%−165.1%, respectively). Compared to the +P treatment, the
Responses of crop yield, P utilization and soil P fractions to different P fertilization regimes
Fertilization treatments had greater impact on crop yield and shoot P uptake in Po-low soil, especially in the oilseed rape season (Table 2, Table 3), as the lower available P content in the soil limits crop growth (Fig. S1) (Wang et al., 2016b). Oilseed rape is particularly sensitive to P deficiency and needs enough quantities of P for optimal growth and development (Hu et al., 2010). In Po-low soil, both rice and oilseed rape yield and P uptake in the -P treatment sharply decreased in the
Conclusions
We conclude that straw addition increased crop yield and P uptake and that cumulative P utilization efficiency increased by a higher percentage in Po-high soil than in Po-low soil. Straw addition promoted microbial biomass pools (MBC and MBP), reduced the MBC:MBP ratio in Po-high soil, and increased the MBC:MBP ratio in Po-low soil, and increased MBP turnover rates and fluxes in both soils. Indeed, the increases in the soil labile and moderately labile P fractions are likely due to MBP turnover
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.
Acknowledgments
This work was supported by the National Key Research and Development Program of China (2020YFD1000904), the China Agriculture Research System of MOF and MARA (CARS-12) and the Fundamental Research Funds for the Central Universities (2662020ZHPY005). The authors would like to thank Mr. Yunqing Yang and Mr. Shaohua Wang for their contributions to the experimental station and Yu Liu and Shuntao Zhang for their assistance in field and laboratory work.
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