Assessment of soil nitrogen and related enzymes as influenced by the incorporation time of field pea cultivated as a catch crop in Alfisol

Application of enzymes as a diagnostic tool for soils as affected by municipal solid wastes

Assessing the relationship between soil enzyme activities (SEAs) and heavy metals (HMs) without any amendment has rarely been conducted in soils contaminated with municipal solid wastes (MSW). Five soil enzymes [dehydrogenase (DHA), alkaline phosphatase (ALP), acid phosphatase (ACP), urease (UR), and nitrate reductase (NR)] have been assessed for HMs bioremediation using Zea mays L. grown in unamended soils that were contaminated with different types of MSW. Pot experiment was conducted for two seasons with soils collected from seven different locations within the MSW site. Experimental soil samples included a control (CA), contaminated by brick kiln wastes (SA1), kitchen and household wastes (SA2), medical wastes (SA3), mixed wastes (SA4), glass wastes (SA5), and metal scrap wastes (SA6). Rhizospheric soils were collected after the harvest of each season to investigate the impact of HMs on SEAs and physicochemical properties of soil. The results revealed an increase in DHA, ALP, and NR activities by 89.30%, 58.03% and 21.98% in SA1. Likewise, enhanced activities for UR (28.26%) and ACP (19.6%) were observed in SA3 and SA5 respectively. Insignificant increase in the macronutrients and organic carbon (OC) were also noted. The increased microbial count and the relatively higher amount of organic matter (OM) in the rhizosphere indicated the role of OM in HMs immobilization. Principal component analysis (PCA) indicated that DHA and NR are the important soil enzymes, underscored by their active involvement in the C and N turnover in the soil. Likewise, correlation analysis showed that DHA and NR activities were positively correlated with copper (Cu) (0.90, p < 0.01; 0.88, p < 0.01), suggesting its participation as a cofactor in enzymatic activities. In contrast, DHA was negatively correlated with cadmium (Cd) (-0.48, p < 0 0.05). Finally, these results indicated that in the absence of exogenous nutrient amendment, the SEAs were governed by OC, available nitrogen (Avl. N), Cu and Cd respectively. The study also highlighted the need for extensive research on SEAs for its utilization as a bioindicator in various soil bioremediation and quality management practices.

Effects of cropping system and fertilization regime on soil phosphorous are mediated by rhizosphere-microbial processes in a semi-arid agroecosystem

In semi-arid regions, soil phosphorus (P) dynamics in cereal-legume intercropping are not yet fully elucidated, particularly in relation to integrated application of fertilizers. To this aim, we investigate the effects of different fertilizers on various P fractions in relation to the rhizosphere-microbial processes in a cowpea/maize intercropping system. Field experiments were conducted during two consecutive years (2016-2017) in a split-plot design by establishing cowpea/maize alone or intercropped onto the main plot, while the sub-plot was treated with four types of fertilization, i.e. no fertilizer addition (control), organic amendment (compost), mineral fertilizers (NPK) and multi-nutrient enriched compost (NPKEC). Our results showed that NPKEC fertilizer increased NaHCO₃-P_i by 69% in maize, 62% in cowpea and 93% in intercropped plots compared to control plots. Similarly, a significant increase in the NaHCO₃-P_o fraction was also recorded with NPKEC treatment in all cropping systems. In case of moderately labile P, NPKEC fertilizer caused the highest increase of NaOH-P_o (12.87 ± 0.50 mg P kg^-1 soil) and NaOH-P_i (22.29 ± 0.83 mg P kg^-1 soil) fractions in intercropped plots. Except for intercropping, NPK application caused an increase in the non-available P fraction (HCl-P_i), while the use of NPKEC decreased the HCl-P_i concentration in all cropping systems, suggesting stronger merits both for intercropping and NPKEC. Surprisingly, maize exhibited substantially higher phosphatases activity compared to cowpea in monoculture amended with compost, implying distinct crop strategies for adaptation under low P conditions. Based on the multi-factor analysis, the close association of NaHCO₃-P with P solubilizing bacteria, root carboxylates and pH indicated that rhizosphere processes are the strongest predictors of immediately available P. Since alkaline phosphatase (ALP) is a P-degrading enzyme of microbial origin, rhizosphere related ALP association may have originated from root-associated microflora promoting P mobilization. Furthermore, the strong association of microbial biomass P (MBP) and acid phosphates (ACP) with NaOH-P fraction indicated moderately available P cycle in soil was mainly driven by microbial-related processes. Factor analysis map and two-way ANOVA confirmed that fertilization regime had a stronger effect on all tested variables compared to cropping system. Altogether, our results suggest that a combination of microbial-rhizosphere processes controls the dynamics of P fertility in semi-arid soils. In the broader context of improving soil P fertility, it is highly recommended the use of environmentally sustainable sources of fertilizer, such as NPKEC, which can enhance the competitive performance of legume-cereal intercropping under semi-arid agroecosystems.

Heavy metal contents and enzymatic activity in soils exposed to the impact of road traffic

The aim of the research was to evaluate the influence of car traffic on the content of selected heavy metals in soil from a park area, and to define the dependency between their content and enzyme activity. Soil samples were collected from 13 points located along a communication route, each 100 m from the next and 50 m from the border of the road. Soil material was obtained from two depths (0-20 cm and 20-40 cm) and analysed for: pH in H₂O and in KCl, OC, and texture by laser method. Total content of heavy metals (Pb, Cd, Zn, Cu, Ni), available phosphorus and the activity of selected enzymes: catalase, dehydrogenase, acidic and alkaline phosphatase were all determined. The examined soils have the texture of loamy sands or sands (USDA 2012), slightly acidic or neutral pH, Organic Carbon (OC) content in the range from 3.50 to 13.80 g kg^-1. The total contents of elements in surface horizons were, in order of decreasing concentrations: Pb>Zn>Cu>Ni>Cd, although in subsurface horizons it was Zn>Pb>Cu>Ni>Cd. Contamination Factor (CF) determined for Ni, Pb, Cd, Zn, Cu reaches higher values in samples from subsurface horizons, which confirms the influence of car traffic on the content of heavy metals in the surrounding soils. The calculated CF shows contamination is moderate for Ni, Cd, Zn and Cu and high for Pb and Cu. The investigated soils may be classified as class IV (low) in terms of available phosphorus. The activity of the examined enzymes was higher in soil samples collected from the 0-20 cm layer than from 20-40 cm. The correlation analysis indicates a significant positive dependency between OC content in soils and enzymatic activity. Principal Component Analysis (PCA) was also performed. Two principal components PC1 and PC2 account for 66.57% of the variability.

Activity of selected enzymes as markers of ecotoxicity in technogenic salinization soils

The activity of enzymes in soil is sensitive to the changes in soil properties affected by biotic and abiotic factors. This study investigates the influence of salinity on some enzymes (catalase CAT, dehydrogenases DEH, alkaline AlP, and acid AcP phosphatase) and pH in 0.01 M CaCl₂, EC_e, the content of total organic carbon, and total nitrogen in technogenic salinization soil next to the soda plant. Seven soil sampling sites were selected (S1-S6) in the area close to the soda plant and C (the control). Based on the enzyme activity, also soil indicators were calculated: the resistance index (RS), enzymatic pH indicator [Formula: see text], the factor of the impact of anthropopressure (IF), the biological index of fertility (BIF), and the indices of biochemical soil activity (BA12 and BA13). The above study did not show one-way changes of the parameters investigated. The relations between the parameters and the activity of catalase, dehydrogenase, alkaline, and acid phosphatase show that they are mostly determined by the state of salinity of the soil environment. The calculated index of resistance (RS), as an effective means of the enzymatic response to environmental stress, facilitated putting the enzymes in the following series: CAT>DEH>AlP>AcP. It shows that catalase and dehydrogenases are most resistant to the anthropogenic factor. The calculated values of BA12 and BA13 indices showed the differences between technogenic salinization soils and the soil sampled from the control. The lowest BIF values were observed at S6 and S3, S4, and C.

Continuous applications of biochar to rice: Effects on nitrogen uptake and utilization

Improving soil quality is critical for increasing rice yield, and biochar could be a beneficial soil amendment for high yield. This study was conducted to determine the effects of continuous (repeated seasonal) applications of biochar on nitrogen (N) uptake and utilization in rice. A fixed field experiment was done in Yongan Town, Hunan Province, China, in six continuous seasons (the early and late rice-growing seasons from 2015 to 2017). Results showed that biochar application did not significantly affect soil N uptake in the first four seasons. The effect of biochar application on fertilizer N uptake was not significant in three of the first four seasons. In the fifth and sixth seasons, biochar application resulted in 14–26% increases in soil N uptake but 19–26% decreases in fertilizer N uptake. Soil N availability did not explain the increased soil N uptake with biochar application. The decreased fertilizer N uptake with biochar application was attributed to both decreased fertilizer N availability and increased N loss through ammonia volatilization. As a consequence of a compensation between the increased soil N uptake and the decreased fertilizer N uptake, the effect of biochar application on total N uptake was not significant in the fifth and sixth seasons. However, biochar application led to 7–11% increases in internal N use efficiency in the fifth and sixth seasons and 6% increase in grain yield in the sixth season. Our study suggests that the effects of repeated seasonal applications of biochar on N uptake and utilization in rice depend on the duration of biochar application. Longer continuous applications of biochar can increase internal N use efficiency and grain yield in rice with insignificant change in total N uptake.