| Soil enzymes, as biotic factors influence the distribution of organic matter (OM) and ultimately play decisive roles in the retention of OM in soil ecosystems. A greenhouse incubation experiment of six months duration was carried out to investigate the activities of oxidative and hydrolytic enzymes and their role in OM decomposition under two different moisture conditions (dry and submerged) and plant residues i.e. rice straw (RS) and green manure (GM) in Brown alkaline also known as Cinnamon and Red acidic soils of China. The study was conducted in a randomized complete block design with ten treatments in triplicates. The RS and GM were used at three rates (0,5and25mg g-1soil; expressed as control (CK), RS1, RS2, GM1and GM2respectively). The soils were maintained at two water levels i.e.25%(W1) and200%(W2). We observed that the activities of soil phenol oxidase and catalase in the Brown soil were2and1.5-folds higher than that in the Red soil. This led to high OM decomposition, as a result,1.23and1.2-folds higher levels of carbon (C) and nitrogen (N) mineralization, and1.23and1.21-folds more dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) than in Red soil, and ultimately low OM. Contrarily, low oxidative enzyme activities in the Red soil diminished the C and N mineralization and dissolved organic matter, as a result1.38-fold more OM was found. Similarly, due to the low activity of phenol oxidase and catalase in the Red soil, the values of reducing sugar and amino acids were2.58and2.22-folds lower, whereas phenolic compounds were1.36-folds higher than in Brown soil. This indicated that as a consequence of low activities of oxidative enzymes in Red soil no favorable conditions for the degradation of phenolic compounds exists, which ultimately diminished the rate of decomposition and the amount of reducing sugars and amino acids. The incorporation of GM exhibited significant (P<0.05) stimulating effects on soil enzymatic and biochemical properties, followed by RS. Water levels also had significant (P<0.05) effects and the addition of high water contents i.e. submerged conditions decreased the decomposition, enzymatic and chemo-biological properties, at the outset of incubation. The alkaline soil pH optimized the oxidative enzymes activity by controlling the substrate availability.We found that under submerged water conditions hydrolytic enzymes i.e. urease and neutral phosphatase are unable to play pertinent roles in the decomposition of soil organic matter (SOM) and showed non-significant correlation (p<0.05) with SOM in both Red and Brown soils. Conversely, oxidative enzymes like phenol oxidase and catalase performed vital parts in the decomposition of SOM and presented significant correlation (p<0.05) with SOM in both Red and Brown soils. We observed that low decomposition of organic inputs, especially lignin, cellulose and hemicelluloses, rich organic inputs, under submerged water conditions, increased the recalcitrant organic polymers i.e. phenolic compounds. This ultimately suppressed the activity of hydrolytic enzymes and disallowed them to play active roles in OM decomposition. We also confirnned that urease and neutral phosphatase enzymes are inept to trigger the decomposition of recalcitrant organic polymers like phenolic compounds, therefore non-significant correlation between urease, neutral phosphatase and phenolic compounds was attained in both Red and Brown soils. Nevertheless, they played significant roles in the biodegradation of easily decomposable organic compounds and exhibited highly significant correlations (p<0.01) to amino acids and reducing sugars in both Red and Brown soils. On the other hand, oxidative enzymes i.e. phenol oxidase and catalase (p<0.01) showed significant relationships with phenolic compounds as well as with amino acids and reducing sugars in both Red and Brown soils. It suggested that, even under rigorous moisture conditions, oxidative enzymes are key components of the pathways involved in the breakdown of complex OM in soils. Our results suggested that high soil enzymatic activity is responsible for active OM decomposition and oxidative enzymes e.g. phenol oxidase and catalase are responsible for the degradation of recalcitrant organic polymers e.g., phenolic compounds and accumulation of OM in soils, consequently act as’enzymatic latch’ even under dogmatic moisture conditions. The water contents and pH are important factors which affect the soil enzymes and therefore decomposition process.To investigate the effect of organic amendments (RS and GM) and water levels (25%and200%) on the soil microbial biomass and microbial population shifts in Red soil of China, an incubation experiment was conducted. The study was laid out in a randomized complete block design with ten treatments in triplicates. The soil microbial biomass and microbial population were determined with the standard customary methods. Results obtained by conventional methods, corroborated with microcalorimetry. We observed that the incorporation of RS and GM, especially at higher rates (25mg g-1soil) significantly (P<0.05) enhanced the soil microbial biomass carbon (MBC), microbial biomass nitrogen (MBN) and microbial biomass phosphorous (MBP) and microbial population i.e. fungi, bacteria and actinomycetes as compared with the control treatment (CK). Comparing the efficacy of two plant residues i.e. GM and RS, we observed that the use of GM exhibited the most significant stimulating effects followed by RS. Water levels i.e.25%(W1) and200%(W2) also had significant (P<0.05) effects; microbial biomass and population decreased with the increase in the water level. To corroborate the results of customary methods with microcalorimetry, the thermodynamic parameters were measured and the power-time curves were recorded for soil samples supplemented with glucose and ammonium sulphate. Microbial growth rate constant k, total heat evolution Q, peak height Pmax and peak time tmax were calculated. Highest Pmax,k and Q were observed in organic amended treatments at water level25%(W1), whereas tmax in control and organic amendments with water level200%(W2). The microbial activity presented by higher growth rate, more peak height, shorter peak time and longer heat dissipation per cell unit indicated that microorganisms under organic amendments had more efficient metabolism, whereas, low microbial activity under control treatments was due to nutrient-deficiency and lack of substrate availability. The microcalorimetric parameters, Pmax,k and Q (positively correlated at p<0.01) and tmax (negatively correlated at p<0.01) significantly correlated with the results of all measured parameters obtained by traditional techniques. We found that all calorimetric parameters Pmax, tmax, Q and k are highly sensitive to many soil intrinsic properties and could be used as indices of soil microbial community shifts and activities.To examine the effect of two contrasting water levels (25%and200%) and organic amendments (RS and GM) on the soil enzymatic (catalase, urease, alkaline phosphatase, neutral phosphatase, phenol oxidase and dehydrogenase), microbiological (MBC, MBN, MBP, bacteria, fungi, actinomycetes) and chemical (SOM, DOC, DON, C mineralization, N mineralization, pH and EC) of Brown soil collected from Henan Providence, China, an incubation study was conducted. The RS and GM were put into pots and mixed thoroughly at two rates (5and25mg g-1soil). The soil was maintained at two water levels i.e.25%(W1) and200%(W2) of water holding capacity with deionized water. The study was conducted with the objective to assess soil enzymatic, microbial and chemical properties by customary methods and results obtained by conventional methods, substantiate with microcalorimetry. The addition of GM and RS, especially at high rates significantly (P<0.05) enhanced the soil enzymatic, microbial and biochemical properties compared to controls. Comparing the effects of two contrasting plant residues, we observed that GM residue had higher significant (P<0.05) stimulating effects on soil enzymatic and biochemical properties than RS. Similarly, water levels i.e.25%(W1) and200%(W2) of water holding capacity also had significant (P<0.05) effect on soil enzymatic and biochemical properties. The water level, W2, had significant (P<0.05) negative effects on all measured enzymatic and biochemical properties i.e. oxidative and hydrolytic enzymes activity, microbial biomass, microbial population, C and N mineralization, dissolved organic matter (DOM), and electrical conductivity (EC), while had positive impact on SOM. To compare the results of conventional methods and to check the sensitivity of microcalorimetry, the thermodynamic parameters, microbial growth rate constant (k), total heat evolution (Q), peak height (Pmax) and peak time (tmax) were calculated. Highest Pmax, k and Q were observed in treatments amended with higher rates of GM residue at water level W1, whereas highest tmax was recorded in CK1(control with water level W1). The microcalorimetric parameters, Pmax, and k were positively correlated, and tmax negatively linked with the results of all measured enzymatic, microbial and biochemical properties at p<0.01. Conversely, Q elucidated non-significant correlation (p<0.05) to urease (0.248), neutral phosphatase (0.281), dehydrogenase (0.291) MBC (0.283), MBP (0.277), DOC (0.269), DON (0.190), SOM (0.284) and pH (0.047). Our results suggested that microcalorimetric parameters Pmax, tmax and k are highly sensitive to many soil intrinsic properties and could be used as indices of soil enzymatic, microbial and biochemical properties in soil ecosystems. Nevertheless, the presence of a non-significant correlation between microcalorimetric parameter Q and urease, neutral phosphatase, dehydrogenase, MBC, MBP, DOC, DON, SOM and pH indicated that microcalorimetric parameters k, Pmax and tmax quantitatively reflect the influence of soil management practices and reforms on soil enzymatic, microbial and chemical properties much better than do Q. |
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