Understanding phosphorus mobilization mechanisms in acidic soil amended with calcium-silicon-magnesium-potassium fertilizer

Calcium-silicon-magnesium-potassium fertilizer (CSMP) is usually used as an amendment to counteract soil acidification caused by historical excessive nitrogen (N) applications. However, the impact of CSMP addition on phosphorus (P) mobilization in acidic soils and the related mechanisms are not fully understood. Specifically, a knowledge gap exists with regards to changes in soil extracellular enzymes that contribute to P release. Such a knowledge gap was investigated by an incubation study with four treatments: i) initial soil (Control), ii) urea (60 mg kg-1) addition (U); iii) CSMP (1%) addition (CSMP) and iv) urea (60 mg kg-1) and CSMP (1%) additions (U + CSMP). Phosphorus mobilization induced by different processes was distinguished by biologically based P extraction. The Langmuir equation, K edge X-ray absorption near-edge structure spectroscopy, and ecoenzyme vector analysis according to the extracellular enzyme activity stoichiometry were deployed to investigate soil P sorption intensity, precipitation species, and microbial-driven turnover of organophosphorus. Results showed that CaCl2 extractable P (or citric acid extractable P) content increased by 63.4% (or 39.2%) in the soil with CSMP addition, compared with the study control. The accelerated mobilization of aluminum (Al)/iron (Fe)-bound P after CSMP addition, indicated by the reduction of the sum of FePO4·2H2O and AlPO4 proportion, contributed to this increase. The decrease of P sorption capacity can also be responsible for it. The CSMP addition increased enzyme extractable P in the soil nearly 7-fold and mitigated the limitations of carbon (C) and P for soil microorganisms (indicated by the enzyme stoichiometry and ecoenzyme vector analysis), suggesting that microbial turnover processes also contribute to P mobilization in amended acidic soil. These findings indicate that the P mobilization in CSMP amended acidic soil not only attributed to both decreasing P sorption capacity and dissolving phosphate precipitation, but also to the increase of the microbial turnover of the organophosphorus pool.

Response of soil micro-food web to nutrient limitation along a subtropical forest restoration

Forest ecosystem productivity and function is strongly influenced by the interaction between soil organisms and their resource use that can be impeded by an imbalance of ecological stoichiometry. Soil microorganisms are known to have an important role in biogeochemical cycling which is strongly influenced by ecological stoichiometry. However, there is limited understanding of how soil micro-food web respond to stoichiometric imbalances during forest restoration. Here, we investigated the effect of forest restoration on soil physio-chemical properties and the structure and function of soil micro-food web along a chronosequence of transformation stages: (i) early stage monoculture plantation of Chinese fir (Cunninghamia lanceolata) comprised of three age classes (5, 10 and 20 years); (ii) mid-stage conifer-broadleaved mixed forest; and (iii) late-stage mixed species broadleaved forest in south China. Results showed that forest restoration from C. lanceolata monocultures to mixed species broadleaved forest significantly increased soil organic carbon and total nitrogen. Soil bacteria, fungi, protists and nematodes abundance increased and the co-occurrence networks of soil biota became more complex and stable along the restoration chronosequence. In contrast, soil nitrogen and phosphorus limitations, particularly phosphorus limitation, increased along the chronosequence. In addition, soil exoenzyme activity suggested that the microbial investment in resource acquisition shifted from C- to nutrient-acquiring enzymes from the earlier to the later restoration stages. Availability of soil resources (e.g., dissolved organic carbon, ammonium, and available phosphate) appeared to have an important role in regulating soil food web composition, structure and stability during forest restoration. We conclude that nutrient limitation, particularly phosphorus limitation, likely has an important role in determining the stability of soil food webs during forest restoration. These findings contribute to our understanding of the relationships between soil nutrient limitation and soil micro-food web, and have implications for carbon sequestration through forest restoration and management in southern China.

Dry-wet cycling area enhances soil ecosystem multifunctionality in the aquatic-terrestrial ecotones of the Caohai Lake in China

The microbial need for nutrient resources can be assessed by soil extracellular enzymes and their stoichiometry. Changes in lake water levels affect land use and nutrient management in the aquatic-terrestrial ecotones of the lakeshore. However, the drivers of changes in microbial nutrient limitation under different inundation gradients in the lake's aquatic-terrestrial ecotones remain unclear. Here, based on vector analysis, we assessed microbial nutrient limitation by studying soil enzyme activities in four different inundation zones (heavy, moderate, mild, and non-inundation) in the aquatic-terrestrial ecotones of Caohai Lake. The findings indicate that inundation conditions significantly influenced the soil properties and enzyme activities. The mean nitrogen and phosphorus acquisition enzymes were higher in both moderate inundation (Mod-inu) and mild inundation (Mil-inu) zone soils, indicating rapid N and P turnover rates in these two zones. However, microorganisms had higher carbon requirements and higher enzyme C:N and vector lengths in heavily inundated compared to lightly inundated. Compared to the non-inundation zone, the microbial phosphorus limitation was found to be most severe in heavy inundation (Hea-inu) and Mod-inu zones. Decreased phosphorus limitation following the inundation weakens could be contributed to improving soil ecosystem multifunctionality. The alterations in the soil extracellular enzymes and stoichiometric characteristics in various inundation zones were primarily influenced by factors such as soil moisture content, available phosphorus, and nitrate nitrogen. Overall, the Mod-inu and Mil-inu zones can better maintain the multifunctionality of the aquatic and terrestrial ecosystems; special attention should be given to the microbial phosphorus limitation in the Hea-inu zone in order to effectively manage nutrients and restore soil ecosystems in the aquatic-terrestrial ecotones.

Determinants of soil carbon- and nitrogen-hydrolyzing enzymes within different afforested lands in central China

Soil organic matter (SOM) decomposition is regulated by a complex set of enzymes. However, the influences of biotic and abiotic factors on spatial variations of soil enzyme activity (EA) within ecosystems remain unresolved. Here, we measured EA at different locations within two afforested lands (coniferous woodland and leguminous shrubland), and simultaneously collected data on soil physico-chemical, vegetation-related, and microbial properties to identify the determinants of EA spatial patterns. The results showed that soil organic C and total N contents were the predominant abiotic factors in regulating absolute EA (EA per unit of oven-dry soil mass) in both afforested lands, while soil pH was the predominant factor in regulating specific EA (EA per unit of microbial biomass (MB)). However, the predominant biotic factors varied with the afforested type: the root biomass and MB were the determinants of EA in the shrubland, whereas the tree distribution, litter and root biomass, and bacterial biomass were the determinants in the woodland. Vegetation-related factors (i.e., litter and root biomass) indirectly influenced soil EA by regulating the soil abiotic factors. Compared with the MB, microbial community composition had a minor impact on EA. The variance of specific EA (EA per unit of MB or SOM) explained by selected factors was much lower than that of absolute EA. In addition, the enzymatic C/N ratio within ecosystems did not follow a general pattern (1:1) observed at a global scale. Our results provide novel experimental insight into ecosystem-level spatial variability of C and N cycling via enzymes, suggesting that soil abiotic factors are more reliable than biotic factors to reflect EA spatial patterns across afforested systems.

Grazing mediates soil microbial activity and litter decomposition in salt marshes

Salt marshes contribute to climate change mitigation because of their great capacity to store organic matter (OM) in soils. Most of the research regarding OM turnover in salt marshes in times of global change focuses on effects of rising temperature and accelerated sea-level rise, while effects of land-use change have gained little attention. The present work investigates the mechanisms by which livestock grazing can affect OM decomposition in salt marsh soils. In a grazing exclusion experiment at the mouth of the Yangtze estuary, China, we assessed soil microbial exo-enzyme activity (EEA) to gain insight into the microbial carbon (C) and nitrogen (N) demand. Additionally, we studied the decomposition of plant litter in soil using the Tea Bag Index (TBI), a widely used standardized litter bag assay to fingerprint soil decomposition dynamics. Based on EEAs, grazing markedly reduced microbial C acquisition, whereas microbial N acquisition was strongly increased. These opposing grazing effects were also evident in the decomposition of standardized plant litter: The decomposition rate constant (k) and the stabilization (S) of litter were not inversely related, as would be expected, but instead both were reduced by livestock grazing. Our data suggest that gazing effects on EEAs and litter decomposition can just partly be explained by grazing-driven soil compaction and resulting lower oxygen availability, which has previously been hypothesized as a main pathway by which grazing can reduce microbial activity in wetland soils. Instead, grazing effects on microbial nutrient demand occurs to be an at least equally important control on soil decomposition processes.

Disentangling the effects of driving forces on soil bacterial and fungal communities under shrub encroachment on the Guizhou Plateau of China

Global shrub encroachment (SE) affects the structure and function of grassland ecosystem. The effects of SE on plant and soil abiotic properties have been well studied; however, little is known about the extent to which driving forces structure soil microbes under SE, especially in subalpine regions of the Guizhou Plateau of China, which is undergoing progressive SE. We investigated the plant factors (viz, plant diversity and relative shrub cover), soil physicochemical properties, enzymatic activities, and microbial communities, quantified microbial element limitations under three encroachment stages, and disentangled the effects sizes of the factors that structure the diversity and composition of soil microbial communities. Redundancy analysis showed that soil factors made a greater contribution than plant factors to shaping the diversity and composition of the soil bacterial community, soil chemical factors made a greater contribution than physical factors both to structuring the diversity and composition of the soil bacterial community and to structuring the composition of the soil fungal community; and soil nutrient stoichiometry made a greater contribution than soil nutrient content to shaping soil bacterial community's diversity and fungal community's composition. In contrast, soil nutrient content made a greater contribution than soil nutrient stoichiometry to shaping the soil bacterial community's composition. The decrease in bacterial community's diversity observed under SE was attributable to increases in the carbon and nitrogen limitations consequent to SE, and the nitrogen limitation had a greater contribution to the soil bacterial community's diversity and composition than did the carbon limitation. These findings provide updated knowledge of the driving forces shaping the diversity and composition of soil microbial communities, which could be crucial for improving microbial prediction models and revealing the element cycling that occurs in SE biomes.