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The biogeochemical interactions between soil nutrients and microbial enzymes in agroecosystems remain poorly understood. The objective of this study was to determine effects of long-term (26 years) application of chemical fertilizers (NPK) or adding manure (M) on the elemental and the enzymatic functional stoichiometric characters in a brown soil. The concentrations of soil elements, i.e. carbon (C), nitrogen (N), phosphorus (P) and sulfur (S) and the activities of soil hydrolases including α (β)‑galactosidase (α-GAL and β-GAL), α (β)‑glucosidase(α-GLU and β-GLU), urease, protease (PR), phosphomonoesterase (PM), Phosphodiesterase (PD) and arylsulfatase (AS) were investigated. Results showed that the NPK additions alone led to a decrease in soil pH and increased the acidification of the soils, while M additions could buffer the acidification. With doubling application rate of M, the concentrations of soil C, N, P and S, and soil enzymes activities increased significantly, but P concentrations increased stronger (doubled) than C and N contents (increased by 30%), leading to shifts in element stoichiometry in response to fertilizer and manure applications. The manure additions led to a relatively constant C:N ratio, a low C:P ratio and a high C:S ratio but lower functional ratios of ln (β-GLU + α-GLU + α-GAL + β-GAL): ln(urease + PR); ln(GLU + GAL): (PM + PD); and ln(GLU + GAL):ln(AS), which implied high availability of P and low availability of S. The comparison of element and enzyme stoichiometry indicated that N and S were limited in the manure plots, and C and P were limited in the NPK and No-NPK fertilizer treatment (WF) plots. The increase in enzyme activities in manure plots was caused primarily by the accumulation of soil nutrients with M treatment and not by a buffering effect on the pH.

Decomposition of soil organic matter (SOM) is operated by microbial extracellular enzymes and is dependent on temperature. It remains unclear how input of labile organics interacts with temperature on SOM decomposition. Soil samples collected from a temperate forest were incubated for 3 months to quantify SOM decomposition and its priming induced by input of 14C-labelled glucose at five temperatures (from 3 to 31 °C with an interval of 7 °C). The activities of eight extracellular enzymes were measured four times during the incubation. Increasing temperature accelerated SOM decomposition with a temperature sensitivity (Q10) value ranging from 1.3 to 4.4. Response of priming to temperature was mediated by the activity and variety of extracellular enzymes temporally: (1) priming increased with increasing temperature at the initial stage; (2) subsequently, priming at lower temperature gradually exceeded those at higher temperature; and (3) finally, priming at high temperature was higher than at low temperatures. At higher temperatures (17–31 °C), N-acquiring enzymes (leucine amino peptidase and N-acetyl-β- d -glucosaminidase) dominated at the early stage of priming, but C-acquiring enzymes (β-glucosidase and xylanase) dominated at later stages. Priming exhibited a hysteresis at low temperatures (3–17 °C) with C-acquiring enzyme (β-galactosidase) dominating at the initial stage, C-and P-acquiring enzymes (α-glucosidase and phosphomonoesterase) at the middle stage, and C-acquiring enzymes (β-galactosidase and xylanase) at the later stage. Consequently, microbial extracellular enzymes were produced in terms of nutrient availability over time depended on temperature. Such acclimation of microbial extracellular enzymes to temperature and nutrient availability should be considered to predict responses of C cycling to global change.

Rapid exposure of anoxic microbial communities to oxygen (O2) can have unpredictable effects, including strong suppression of their enzymatic activity. Nonetheless, most medium- and long-term incubation studies on soil organic matter transformations fail to consider aeration effects during sample post-processing and/or assays. Moreover, it remains unclear whether anoxic enzymatic systems are adapted to quick switch to oxic conditions. We evaluated the effects of short-term (2-h oxic (+O2) vs. anoxic (–O2) assays) and medium-term aeration (after 10-day oxic vs. anoxic pre-incubation) on the kinetic parameters (Vmax, Km) of phosphomonoesterase, β-glucosidase, and leucine aminopeptidase in top bulk, rooted, and bottom bulk paddy soil of flooded rice mesocosms. We hypothesized contrasting short- and medium-term responses of hydrolytic enzyme activities to aeration (i) a negative short-term effect caused by reactive O2 species toxicity and/or other mechanisms, and (ii) adaptation of anoxic microbial communities to medium-term aeration reducing the impact of ongoing O2 exposure. Overall, 2-h aeration suppressed Vmax values by 7–43% and catalytic efficiency Ka (Vmax/Km) by 3–22%, and extended the substrate turnover time Tt (7–33%) of three tested enzymes in all soil compartments pre-incubated without O2. In contrast, no short-term suppressive effect of O2 was observed on three tested enzymes after oxic pre-incubation. Medium-term aeration increased Vmax (by 12–253%) and Ka (by 3–78%) of the enzymes and shortened Tt (4–42%) as compared to the anoxic counterpart. These findings support our hypothesis about anoxic microbial community adaptation over the medium-term aeration. Accordingly, the sensitivity of anoxic hydrolytic enzymes to a short-term O2 exposure and the O2 adaptation mechanisms require strong consideration (i) for enzyme assays of anoxic soils and (ii) for understanding the soil organic matter dynamics in environments with O2 fluctuations.

Understanding soil phosphorus (P) availability can benefit from characterizing P pools and processes governing P distribution among such pools. Hedley P fractionation has been widely used to characterize P pools, whereas soil phosphomonoesterase (PME) activity assays have been used to assess potential rates of organic P (Po) mineralization to available inorganic P (Pi). We evaluated 1) the extent to which P fractions are consistent with the theorized P pools and 2) relationships of PME activity with P fractions using observations compiled across diverse (agro)ecosystems. We hypothesized that P fractions are differentiated based on the form (i.e. organic and inorganic) and availability of P pools they estimate. Given that Pi is product and Po is substrate for PME, we further hypothesized that PME activity associates negatively with Pi fractions representing available P pool and positively with Po fractions. Factor analysis grouped measured P fractions into three pools: non-occluded Pi, soil organic P, and secondary Pi. These groupings generally supported the differentiation of P fractions by form (Pi vs Po) and availability of P pools they are commonly used to estimate but were also sensitive to the degree of soil weathering. Structural equation modeling supported biochemical mechanisms of product inhibition and substrate induction of PME activity and also identified an influence of weathering degree on relationships of specific measured P fractions with PME activity. This work demonstrates that evaluating PME activity in conjunction with P fractions can be a useful framework to better understand soil P cycling. Incorporating additional variables—such as Po fractionated physically or enzymatically, and phosphodiesterase activity—into the current statistical framework may further improve understanding of how soil P cycling determines P availability.

In soil enzyme assays, buffers are most commonly employed to constrain the assay to a specific pH at which activity is assumed to be maximized (i.e., pH optima). The assumed pH optima prescribed for the assay of soil β-glucosidase and phosphomonoesterase activities are based on early studies from a limited set of soils and have subsequently been found to be invalid in broader contexts. Here, we used the USDA-NRCS national soil survey database to preselect soils with a broad range of edaphic characteristics—namely soil pH, organic matter content, and clay content—to robustly assess pH optima of β-glucosidase and phosphomonoesterase. β-Glucosidase pH optima occurred within a narrow range and were independent of soil or climatic properties, including soil pH. Conversely, phosphomonoesterase pH optima showed a broad range of values that were closely linked to soil pH. For most samples, the mismatch between the prescribed and optimal pH (maximal activity) corresponded to activity differences of

The aim of this work was to evaluate the effects of vegetation and hydrocarbon (HC) contamination on the development of soil chemical and biological status during rhizoremediation of fuel oil contamination with the legume Galega orientalis . Uncontaminated and unvegetated references monitored in parallel with the rhizoremediation treatment enabled the identification of the partial effects. A 21-week greenhouse experiment simulated one growing season with a single 3000 ppm contamination event in the beginning. For a comprehensive view of the restoration process, the following soil parameters were assessed by ten destructive samplings at increasing intervals: plant growth, HC content, pH, C/N ratio, DNA content, culturable oil-degrader numbers and six enzymatic activities. After 21 weeks, 90% and 87% of the initial HC load was depleted in the rhizoremediation treatment and the unvegetated reference, respectively. Contamination retarded the growth of Galega ; a majority of the hydrocarbons were degraded by the indigenous soil microbial community by week 6, when the legume seedlings were still very small. In the end of the experiment, when HC contamination had decreased to the clean soil threshold level, Galega biomass in the rhizoremediation treatment reached that of the uncontaminated reference. In contrast, fuel oil stimulated the growth and activity of the soil microbial community and may have masked vegetation-associated changes. The amounts of total and oil-degrader micro-organisms mirrored the HC degradation curve and seem an efficient and ecologically relevant tool to monitor the biodegradation process of light HC contamination. No negative HC effects were seen in soil enzymatic activities either, but aminopeptidases were induced by contamination. Vegetation-associated upward trend was observed for aminopeptidases and phosphomonoesterase. Biological HC degradation raised soil pH. Part of the fuel oil carbon remained in the soil, perhaps assimilated by the degrader micro-organisms. The results of this study support the theory of high soil resilience to moderate contamination with light hydrocarbons, demonstrate the power of microbiological methods in monitoring bioremediation, and back up the current legislative clean soil threshold level for hydrocarbon contamination.

Soil organic phosphorus contributes to the nutrition of tropical trees, but is not accounted for in standard soil phosphorus tests. Plants and microbes can release organic anions to solubilize organic phosphorus from soil surfaces, and synthesize phosphatases to release inorganic phosphate from the solubilized compounds. We developed a procedure to estimate bioavailable organic phosphorus in tropical forest soils by simulating the secretion processes of organic acids and phosphatases. Five lowland tropical forest soils with contrasting properties (pH 4.4–6.1, total P 86–429 mg P kg− 1) were extracted with 2 mM citric acid (i.e., 10 μmol g− 1, approximating rhizosphere concentrations) adjusted to soil pH in a 4:1 solution to soil ratio for 1 h. Three phosphatase enzymes were then added to the soil extract to determine the forms of hydrolysable organic phosphorus. Total phosphorus extracted by the procedure ranged between 3.22 and 8.06 mg P kg− 1 (mean 5.55 ± 0.42 mg P kg− 1), of which on average three quarters was unreactive phosphorus (i.e., organic phosphorus plus inorganic polyphosphate). Of the enzyme-hydrolysable unreactive phosphorus, 28% was simple phosphomonoesters hydrolyzed by phosphomonoesterase from bovine intestinal mucosa, a further 18% was phosphodiesters hydrolyzed by a combination of nuclease from Penicillium citrinum and phosphomonoesterase, and the remaining 51% was hydrolyzed by a broad-spectrum phytase from wheat. We conclude that soil organic phosphorus can be solubilized and hydrolyzed by a combination of organic acids and phosphatase enzymes in lowland tropical forest soils, indicating that this pathway could make a significant contribution to biological phosphorus acquisition in tropical forests. Furthermore, we have developed a method that can be used to assess the bioavailability of this soil organic phosphorus., Graphical abstract Image 1, Highlights • Development of a method to quantify biologically-available soil organic phosphorus • Extraction conditions standardised to those which best approximate soil environment • Organic phosphorus is an important phosphorus source for tropical forest biota