Soil and microbial C:N:P stoichiometries play vital roles in regulating P transformation in agricultural ecosystems:A review

Stoichiometry plays a crucial role in biogeochemical cycles and can modulate soil nutrient availability and functions. In agricultural ecosystems,phosphorus(P) fertilizers(organic or chemical) are often applied to achieve high crop yields. However, P is readily fixed by soil particles, leading to low P use efficiency. Therefore, understanding the role of carbon:nitrogen:P stoichiometries of soil and microorganisms in soil P transformation is of great significance for P management in agriculture. This paper provides a comprehensive review of the recent research on stoichiometry effect on soil P transformation in agricultural ecosystems. Soil microorganisms play an important role in the transformation of soil non-labile inorganic P to microbial biomass P by regulating microbial biomass stoichiometry. They also mobilize soil unavailable organic P into available P by changing ecoenzyme stoichiometry. Organic materials, such as manure and straw, play an important role in promoting the transformation of insoluble P into available P as well. Additionally, periphytic biofilms can reduce P loss from rice field ecosystems. Agricultural stoichiometries are different from those of natural ecosystems and thereby should receive more attention due to the influences of anthropogenic factors. Therefore, it is necessary to conduct further stoichiometry research on the soil biochemical mechanisms underlying P transformation in agricultural ecosystems. In conclusion, understanding stoichiometry impact on soil P transformation is crucial for P management in agricultural ecosystems.

Responses of soil phosphorus pools accompanied with carbon composition and microorganism changes to phosphorus-input reduction in paddy soils

In rice-wheat rotation systems, changes in soil phosphorus(P) pools and microorganisms in rice-growing seasons have been studied;however, further investigations are required to test whether these indexes exhibit different responses in wheat-growing seasons. Additionally, such studies need to include potential variations in soil carbon(C) structure and microbial community composition. In this study, a long-term rice-wheat rotation P-input reduction experiment was conducted to observe the variations in soil P pools and C composition in the 7th wheat season and to investigate the responses of soil enzyme activity and microbial communities. Four P fertilization treatments were included in the experiment, i.e., P application for rice season only(PR), for wheat season only(PW), and for both rice and wheat seasons(PR+W) and no P application in either season(Pzero). Compared with PR+W treatment, Pzero treatment significantly decreased(P < 0.05) labile and stable P pools. Different P fertilization regimes altered soil microbial community composition and enzyme activity, whereas C composition did not vary. However, PW treatment resulted in relatively more O-alkyl-C than PR treatment and the highest number of microorganisms. Besides, the higher ratios of fungi/bacteria and Gram-positive bactetia/Gram-negative bactetia were related to labile C pools, particularly O-alkyl-C, as opposed to recalcitrant C. Our results clarified the status of soil P pools, C chemistry, and the response of microorganisms under dry-farming conditions in the P input-reduced rice-wheat rotation system.

Using diffusive gradients in thin films technique for in-situ measurement of labile phosphorus around Oryza sativa L. roots in flooded paddy soils

Behavior of phosphorus(P) in flooded rice soil is controlled by iron(Fe) redox cycling in root-zone. In this study, we applied a novel approach—the diffusive gradients in thin films(DGT) technique—for investigating the in-situ distribution of labile phosphorus(P) and Fe in close proximity to Asian rice(Oryza sativa L.) roots at submillimeter to millimeter spatial resolutions during the seedling and booting stages. We conducted a seven-year field experiment under rice-wheat rotation with different P fertilizer treatments. The results showed a significant and strong positive relationship of the average DGT-labile P concentration with soil Olsen P(R2= 0.77, P < 0.01) and with rice total P concentration(R^(2)= 0.62, P < 0.05). Furthermore, results on one-and two-dimensional changes of DGT-labile P indicated that fertilization only in the wheat season produced sufficient amounts of labile P in the flooded paddy soils, similar to when fertilizer was applied only in the rice season;dissolved P concentrations, however, were lower. A co-occurrence and significant positive correlation(P < 0.01) between DGT-labile P and Fe indicated Fe-coupled mobilization of P in flooded paddy soils. These results collectively indicated that the DGT technique provided information on in-situ distribution of labile P and its variability in close proximity to rice roots. This suggests that the DGT technique can improve our understanding of in-situ and high-resolution labile P processes in paddy soils and can provide useful information for optimizing P fertilization.

Long-term biochar addition significantly decreases rice rhizosphere available phosphorus and its release risk to the environment

Phosphorus(P)availability,diffusion,and resupply processes can be altered by biochar addition in flooded rice rhizosphere,which controls the risk of P release to the environment.However,there are few in-situ investigations of these rhizospheric processes and effects.To explore the effects of biochar addition on soil P availability,high-resolution dialysis(HR-Peeper),diffusive gradients in thin films(DGT),and zymography techniques were used to provide direct evidence in the rice rhizosphere at the sub-millimeter scale.Long-term(9-years)field and greenhouse pot experiments demonstrated that biochar addition notably decreased the soluble/labile P and Fe concentrations in rice rhizosphere(vs.no biochar addition;CK)based on the results of Peeper,DGT,and two-dimensional imaging of labile P fluxes.DGT-induced fluxes in the soil/sediment(DIFS)model and sediment P release risk index(SPRRI)further indicated that biochar addition decreased the diffusion and resupply capacity of P from soil solid to the solution,thereby decreasing P release risk to the environment.These processes were dominated by Fe redox cycling and the hydrolysis of Al(hydro)oxides that greatly increased the unavailable P(Ca-P and residual-P).Additionally,greenhouse pot experiments(without additional biochar)showed that the previous long-term biochar addition significantly increased soil phosphatase activity,due to an adaptive-enhancing response to P decrease in the rhizosphere zone.The in-situ study on the biogeochemical reactions of P in the rice rhizosphere may provide a new and direct perspective to better evaluate the biochar addition and potential benefits to agricultural soils.

Seven years of biochar amendment has a negligible effect on soil available P and a progressive effect on organic C in paddy soils

Little attention has been paid to how long-term application of crop straw and its biochar affects soil phosphorus(P)transformation and carbon(C)fractions.We conducted a 7-year field experiment including control treatment(chemical fertilizer only,CK),straw return(2.25 t ha^(−1)),and different amounts of biochar addition(11.25 t ha^(−1)(0.5%BC)and 22.5 t ha^(−1)(1.0%BC),to investigate influence of these amendments on soil C structure,P fractions,and their interaction with microorganisms.The 13C nuclear magnetic resonance and soil P sequence fractionation were applied to capture changes of soil C compositions and P pool.Compared to CK,straw and biochar amendments decreased alkyl C/O-alkyl C,which is conducive to increased soil organic C.The 0.5%BC and 1.0%BC treatments enhanced recalcitrant aromatic C by 69.0%and 131%,respectively.Compared to CK(101.2±33.32 mg kg^(−1)),the 0.5%BC and 1.0%BC treatments had a negligible effect on soil available P,while negative effects were observed in straw treatment(59.79±9.023 mg kg^(−1)).Straw and biochar amendments increased primary P and occluded P,whereas had negligible effect on organic P.Redundancy analysis and correlation analysis indi-cated that C compositions and P pool correlated to microbial community composition and enzyme activities,and aromatic C was the most related factor.Moreover,structural equation modeling indicated available P was most related to phosphatase activity and C composition.Our findings reveal the changes of soil P and C response under long-term crop straw and its biochar amendment,and can contribute toward improving understanding of the effect of biochar and straw return in future agriculture management.

Effects of temperature change and tree species composition on N20 and NO emissions in acidic forest soils of subtropical China

Tree species and temperature change arising from seasonal variation or global warming are two important factors influencing N2O and NO emissions from forest soils. However, few studies have examined the effects of temperatures （5-35℃） on the emissions of forest soil N2O and NO in typical subtropical region. A short-term laboratory experiment was carried out to investigate the influence of temperature changes （5-35℃） on soil N2O and NO emissions under aerobic conditions in two contrasting （broad-leaved and coniferous） subtropical acidic forest types in China. The results showed that the temporal pattern of N2O and NO emissions between the three lower temperatures （5℃, 15℃, and 25℃） and 35℃ was significantly different for both broad-leaved and coniferous forest soils. The effects of temperature on soil N2O and NO emission rates varied between broad-leaved and coniferous forest soils. Both N2O and NO emissions increased exponentially with an increase in temperature in the broad-leaved forest soil. However, N2O and NO emissions in the coniferous forest soil were not sensitive to temperature change between 5℃ and 25℃. N2O and NO emission rates were significantly higher in the broad-leaved forest soil as compared with the coniferous forest soil at all incubation temperatures except 5℃. These results suggest that the broad-leaved forest could contribute more N2O and NO emissions than the coniferous forest for most of the year in the subtropical region of China.

Effect of P fertilizer reduction regime on soil Olsen-P, root Fe-plaque P, and rice P uptake in rice-wheat rotation paddy fields

In agricultural systems, it is vital to use limited yet optimal phosphorus(P) resources, because excessive P fertilizer application leads to the accumulation of P in soil, increasing the risk of environmental pollution and causing the waste and exhaustion of P resources. In a rice-wheat rotation system, omitting P fertilizer application in the rice-growing season is a good alternative;however, how this P fertilization reduction influences changes in P in the soil-root-aboveground system is unclear. In this study, after a seven-year rice-wheat rotation at the Yixing(YX) and Changshu(CS) sampling sites, China, compared with P fertilization in rice-and wheat-growing seasons(PR+W), reduced P fertilization(no P fertilizer application in either season, P0;P fertilization only in wheat-growing seasons, PW;and P fertilization only in rice-growing seasons, PR) did not result in substantial variation in crop biomass. The PW treatment did not reduce crop total P, root iron(Fe)-plaque P, and soil Olsen-P at three stages of rice growth(seedling, booting, and harvesting stages) at the YX and CS sites. In contrast, concentrations of soil Olsen-P, aboveground crop total P, and root Fe-plaque P decreased in the P0 treatment by 45.8%–81.0%,24.6%–30.9%, and 45.6%–73.4%, respectively. In addition, a significant negative correlation was observed between the root Fe-plaque P and crop biomass at the two sites. Significant positive correlations were also observed between root Fe-plaque P and root total P, crop total P, and soil Olsen-P. In addition, the results of a redundancy analysis revealed that soil alkaline phosphatase(ALP) played a major role in the supply of P in soil, and was closely associated with root Fe-plaque P. The results of this study will enhance the understanding of the changes in P in the soil-root-aboveground system, particularly under P fertilizer reduction regimes.

Effects of temperature change and tree species composition on N_2O and NO emissions in acidic forest soils of subtropical China

Tree species and temperature change arising from seasonal variation or global warming are two important factors influencing N2O and NO emissions from forest soils. However, few studies have examined the effects of temperatures(5–35℃) on the emissions of forest soil N2O and NO in typical subtropical region. A short-term laboratory experiment was carried out to investigate the influence of temperature changes(5–35℃) on soil N2O and NO emissions under aerobic conditions in two contrasting(broad-leaved and coniferous) subtropical acidic forest types in China. The results showed that the temporal pattern of N2O and NO emissions between the three lower temperatures(5℃, 15℃, and 25℃) and 35℃ was significantly different for both broad-leaved and coniferous forest soils. The effects of temperature on soil N2O and NO emission rates varied between broad-leaved and coniferous forest soils. Both N2O and NO emissions increased exponentially with an increase in temperature in the broad-leaved forest soil. However, N2O and NO emissions in the coniferous forest soil were not sensitive to temperature change between 5℃ and 25℃. N2O and NO emission rates were significantly higher in the broad-leaved forest soil as compared with the coniferous forest soil at all incubation temperatures except 5℃. These results suggest that the broad-leaved forest could contribute more N2O and NO emissions than the coniferous forest for most of the year in the subtropical region of China.