Long-term impact of organic and inorganic fertilizers on soil enzyme activities in cotton fields

Introduction

Soil enzymes are the biochemical workforce of terrestrial ecosystems. Dehydrogenase reflects overall microbial oxidative metabolism, urease governs nitrogen mineralisation from urea and organic residues, alkaline phosphatase releases bound phosphorus, and βglucosidase breaks down cellulose into glucose that fuels microbial communities [1] . Together, these enzymes determine how efficiently organic inputs are converted into forms that crop roots can absorb. When synthetic fertilisers dominate the nutrient supply, the soil's own enzyme machinery can atrophy over time because microbes no longer need to decompose organic substrates for carbon and energy [2] . Cotton is one of Egypt's most important fibre crops, receiving on average 180 kg N/ha across the Nile Valley cotton belt well above the blanket recommendation [3] . Long-term monoculture under heavy NPK dosing has led to declining soil organic carbon in several alluvial regions of the Nile Valley [4] . Yet quantifying the enzyme response requires experiments sustained over many years, because short-term trials cannot separate seasonal fluctuation from a genuine trend. The Assiut long-term fertiliser trial, initiated in 2009, offers a 14-year record of continuous cotton cultivation under five contrasting nutrient strategies. This research used that platform to test whether integrated organic-inorganic management preserves or enhances the enzyme pool compared with either organic or inorganic inputs applied alone.

Material and Methods

Study area and baseline soil

Material

Five treatments Control (no fertiliser), FYM (10 t/ha/yr), NPK (120:60:40 kg/ha as urea, DAP, MOP), FYM + NPK (half dose each), and vermicompost (5 t/ha/yr) have been applied annually since summer 2009 in a randomised complete block design with four replications. Plot size is 6 m × 5 m. Cotton variety Giza 94 (G. hirsutum) is sown each year in March and picked in September-October. For this research, composite soil samples (0-20 cm, five cores per plot) were collected within one week after the September 2023 harvest [5] .

Methods

Dehydrogenase was assayed by the TTC reduction method [6] , urease by the buffered urea method [7] , alkaline phosphatase by the p-nitrophenyl phosphate method, and βglucosidase by the p-nitrophenyl-β-D-glucoside method [8] . Soil organic carbon was determined by Walkley-Black, Microbial Biomass Carbon (MBC) by chloroform fumigation extraction [9] . Data were subjected to one-way ANOVA followed by Tukey's HSD at p<0.05 using SAS v9.4. Pearson correlations were computed between enzyme activities and soil chemical/biological properties. All four enzymes responded positively to organic amendments. The FYM + NPK treatment consistently produced the highest activities, followed closely by vermicompost. NPK alone raised enzyme levels only marginally above the unfertilised control a difference that was statistically non-significant for urease and βglucosidase.

Results

Comprehensive interpretation

The integrated FYM + NPK treatment outperformed all others because it supplied both labile carbon (from FYM) to sustain microbial populations and mineral nutrients (from NPK) to meet immediate crop demand. This dual action maintained microbial biomass at 348 µg C/g more than double the control and drove enzyme activities 50-70% above unfertilised soil.

Discussion

These findings support the broader conclusion of Dick et al. [10] that soil biological indicators respond to management more sensitively than chemical ones. The 68% increase in dehydrogenase under FYM + NPK matches the 55-80% range documented in Nile Valley long-term experiments by Manna et al. [11] . That pure NPK failed to elevate enzyme activities meaningfully reinforces the idea that mineral fertilisers feed the plant but starve the soil biota a concern raised by Nannipieri et al. [2] in Mediterranean arable systems.

The strong correlations between organic carbon and all four enzymes (r>0.87) confirm that carbon availability is the master variable controlling biological activity. Vermicompost, despite being applied at only half the carbon load of FYM, produced statistically similar enzyme responses probably because its humic fraction is more readily assimilable by microbial communities [12] . This makes vermicompost attractive where FYM supply is limited. A practical caveat is that enzyme measurements are timesensitive. Sampling one week after harvest captures postseason residual activity, but peak enzyme expression likely occurs during the irrigation season when soil moisture and temperature are both optimal. Future sampling at multiple growth stages would give a more complete picture of seasonal enzyme dynamics.

Conclusion

Within the context of Egypt's cotton belt, where continuous monoculture and heavy NPK use have depleted soil biological health in many districts, this research offers a clear recommendation. After 14 years, plots receiving integrated organic-inorganic management showed enzyme activities 50-70% higher and organic carbon nearly double that of unfertilised controls. The data highlight that combining FYM or vermicompost with mineral fertilisers is the most effective strategy for sustaining the enzyme-driven nutrient cycling that underpins long-term cotton productivity. As a forward-looking note, linking these enzyme data with cotton fibre yield trends could quantify the economic value of maintaining soil biological capital.

References

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