Effect of integrated nitrogen and phosphorus management on soil enzymatic activities

Introduction

Soil biological activity plays a crucial role in maintaining soil fertility and ecosystem sustainability. Soil enzymes catalyze biochemical reactions involved in organic matter decomposition and nutrient transformation and are therefore considered reliable indicators of soil health. Among them, dehydrogenase activity reflects total microbial oxidative activity, while urease plays a key role in nitrogen mineralization by catalyzing the hydrolysis of urea into ammonium. Nutrient management practices strongly influence soil enzymatic activity by altering organic carbon availability, microbial biomass, and soil physicochemical conditions. Excessive dependence on inorganic fertilizers may adversely affect soil biological properties, whereas integrated nutrient management combining inorganic fertilizers with organic sources and bio-inputs has been shown to enhance soil microbial activity. Nitrogen and phosphorus are primary nutrients that govern crop productivity as well as microbial metabolism. However, information on their combined influence on soil enzymatic activities under integrated management practices remains limited. Therefore, the present investigation was undertaken to study the influence of nitrogen and phosphorus management practices on soil dehydrogenase and urease activities at different crop growth stages.

Materials and Methods

A field experiment was conducted during the agricultural years 2022-23 and 2023-24 in a factorial randomized block design with three replications. The treatments consisted of six nitrogen management options involving varying proportions of recommended nitrogen dose through inorganic fertilizers, organic sources, and bio-inputs, and four phosphorus management options comprising basal and split applications of recommended phosphorus doses. Soil samples were collected from each treatment plot at 30 DAT, 60 DAT, and at harvest. Dehydrogenase activity was estimated by measuring the reduction of 2,3,5-triphenyl tetrazolium chloride to triphenyl formazan following the procedure described by Casida et al. (1964) [1] and Tabatabai (1994) [8] . Urease activity was determined by quantifying ammonium released from urea hydrolysis as per the method outlined by Tabatabai and Bremner (1972) [9] . These enzymes are widely recognized as indicators of soil microbial activity and nitrogen transformation processes (Dick, 1997; Nannipieri et al., 2012) [2,7] . Year-wise data were pooled after testing for homogeneity of variance and statistically analyzed to assess treatment effects.

Results and Discussion

Dehydrogenase activity: Data on dehydrogenase activity (Table 1) indicated significant differences among nitrogen and phosphorus management practices at all stages of crop growth. Among nitrogen management options, treatment T5 recorded the highest pooled dehydrogenase activity at 30 DAT (77.59 µg TPF g⁻¹ soil day⁻¹), 60 DAT (108.46 µg TPF g⁻¹ soil day⁻¹), and at harvest (56.87 µg TPF g⁻¹ soil day⁻¹). The enhanced activity may be attributed to improved microbial proliferation and availability of organic substrates supplied through integrated nutrient sources. Phosphorus management involving 100% recommended dose of phosphorus (basal) recorded significantly higher dehydrogenase activity compared to reduced phosphorus levels. The increased enzyme activity under integrated nutrient management corroborates the findings of Nannipieri et al. (2003), who reported that organic amendments stimulate soil microbial activity and enzyme synthesis. Interaction effects between nitrogen and phosphorus management were non-significant.

Urease activity

Urease activity data (Table 2) followed trends similar to dehydrogenase activity. Nitrogen management treatment T5 recorded the highest pooled urease activity at 30 DAT (52.91 µg NH₄⁺ g⁻¹ soil h⁻¹), 60 DAT (104.02 µg NH₄⁺ g⁻¹ soil h⁻¹), and at harvest (38.86 µg NH₄⁺ g⁻¹ soil h⁻¹). Increased urease activity under integrated nutrient management could be due to enhanced microbial biomass and greater availability of urea substrates. Among phosphorus management options, 100% RDP (basal) resulted in significantly higher urease activity across all growth stages. These results are in agreement with Dick et al. (1996), who reported that integrated nutrient management practices improve enzyme-mediated nitrogen transformations in soil. Interaction effects were non-significant.

Conclusion

The study demonstrated that integrated nitrogen and phosphorus management practices significantly enhanced soil dehydrogenase and urease activities throughout the crop growth period. Application of recommended nutrient doses in combination with organic sources proved superior in improving soil microbial activity and soil biological health. Adoption of integrated nutrient management practices is essential for sustaining soil enzymatic activity and long-term soil fertility under intensive cropping systems.

References

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