Less mineral nitrogen, stronger reserves: reading a conservation field

A three-year rice–wheat–green gram trial shows why mineral nitrogen alone can miss the crop's supply under residue retention.

This SoilBeat article draws on research by Siddhartha Mukherjee, Dibyendu Sarkar, Sahely Kanthal and Biswapati Mandal, published in Scientific Reports. The findings are theirs; the practical interpretation below is SoilBeat’s. Read the original research.

The agronomic problem

Nitrogen management often starts with a simple question: how much plant-available N is in the soil? In conventionally managed fields, nitrate and ammonium tests can be useful snapshots. But in residue-retaining, reduced-tillage systems, that snapshot may miss an important part of the supply chain.

Crop residues add carbon as well as nutrients. Microbes use that carbon and may temporarily immobilize mineral N. At the same time, reduced disturbance can protect organic matter inside aggregates and build organic-N pools that mineralize gradually. A soil can therefore contain less immediately extractable inorganic N while maintaining—or even improving—crop performance. That is not a contradiction. It reflects different nitrogen forms and turnover rates.

A field study from India’s lower Indo-Gangetic Plain examined that balance in a rice–wheat–green gram rotation. Its practical value is not a universal conservation-agriculture recipe. It is a clearer explanation of why nutrient decisions should track both short-term N supply and longer-term N stabilization.

What the researchers tested

The experiment used a split-plot design with three tillage systems: conventional, reduced, and zero tillage. Each was combined with five rice-residue and fertilizer programs: no retained residue with 100% of recommended NPK; full residue with 50% NPK; full residue with 75% NPK; half residue with 100% NPK; and half residue with 75% NPK.

That created 15 treatments, replicated three times in 10 × 20 meter plots. The subtropical, humid site had a clay-loam Inceptisol with near-neutral pH. The rotation ran for three years, covering nine crop seasons. Rice residue was retained ahead of the following winter crop, and green gram added a legume phase.

After the third rotation, the researchers measured total, organic, and inorganic soil N. They separated organic N into hydrolyzable fractions—amino-acid N, amino-sugar N, hydrolyzable ammonium, serine + threonine, and an unidentified fraction—and non-hydrolyzable N. They also measured a phosphate-buffer-extractable pool intended to represent readily mineralizable N, plus crop yield, plant N, and grain protein.

The main finding: less mobile N did not mean less usable N

Across treatments, roughly 92% of measured soil N was organic and about 8% was inorganic. Zero tillage produced 17% more organic N than conventional tillage, while conventional tillage had 36% more inorganic N than zero or reduced tillage.

That distinction matters. Inorganic nitrate and ammonium are immediately relevant to roots, but they are also vulnerable to leaching, volatilization, and denitrification. Organic N turns over at different speeds. Some compounds are relatively resistant; others can be depolymerized and mineralized during the crop cycle.

The strongest total and organic-N storage occurred under zero tillage with full residue retention and 75% NPK. Meanwhile, the highest hydrolyzable and phosphate-buffer-extractable N occurred under zero tillage with half residue and full NPK. These were not the same treatment, which exposes the central tradeoff: the program that stores the most N is not necessarily the program that supplies the largest near-term pool.

Rice-equivalent system yield ranged from 9.60 to 13.24 metric tons per hectare. On average, the conservation-agriculture treatments improved crop performance and mean grain protein by about 5–7% over conventional tillage with no retained residue and full NPK.

Most importantly, hydrolyzable N and the phosphate-buffer-extractable pool were positively associated with yield and protein. Total N, inorganic N, and non-hydrolyzable N were not. The result suggests that a biologically active organic reserve described crop performance better than a simple inventory of total or mobile N in this system.

Why residue amount and fertilizer rate interacted

The retained rice residue had a high C:N ratio. Microbes decomposing that carbon require N, so full residue retention increased immobilization, especially during early adoption. With only 50% of recommended NPK, that immobilization could restrict short-term availability. At the other extreme, full fertilizer without residue produced more inorganic N but less organic-N stabilization.

The intermediate packages—zero or reduced tillage with 50–100% residue and 75–100% NPK—appeared to balance these processes. Mineral fertilizer helped cover near-term crop demand and microbial immobilization. Residues and the green-gram phase supplied organic inputs. Reduced disturbance limited aggregate disruption and organic-matter oxidation.

The plausible mechanism is therefore not that conservation agriculture creates nitrogen. It changes timing and partitioning: crop residue, root inputs, and legume-derived N enter microbial and organic pools; some becomes protected; some remains potentially mineralizable; and less stays as a transient mineral surplus.

That mechanism is consistent with the measured fractions, but several parts remain inferential. The study did not directly quantify ammonia loss, nitrate leaching, nitrous oxide, dinitrogen, or deep movement. It also did not complete a full N balance.

What this means for soil and plant testing

A low nitrate or ammonium result can mean different things. It may indicate true shortage, recent crop uptake, leaching, gaseous loss, temporary microbial immobilization, or transfer into organic pools. The correct interpretation depends on sampling time, moisture, temperature, residue quality, root activity, and crop demand.

For conservation systems, a stronger diagnostic sequence is to combine:

• soil texture, pH, organic carbon, moisture, and routine mineral N;

• residue amount, C:N ratio, placement, and decomposition stage;

• fertilizer source, rate, timing, and placement;

• tissue or sap N measured at a crop-relevant stage;

• canopy and field observations interpreted with weather;

• yield, protein or quality, residual N, and economics after harvest.

Potentially mineralizable-N tests may add value, but methods differ and regional calibration is essential. The phosphate-buffer extraction used in this study is not a universal recommendation standard. Any new test should earn its place by improving prediction or management decisions in local soils and rotations.

Practical implications—and important limits

The 75% NPK treatments make fertilizer reduction an obvious question, but they do not prove that every conservation field can cut NPK by 25%. The treatment factor reduced N, P, and K together, so the design does not isolate nitrogen alone. Residue supply, fertilizer value, tillage equipment, labor, establishment risk, and market prices were not combined in a full profitability analysis.

The study also represents one humid, clay-loam site and the first three years of system transition. Coarse soils, colder climates, dryland systems, different residues, or rotations without a legume may respond differently. Full residue retention with a large C:N ratio can create an early N penalty unless nutrient supply and placement are synchronized.

The supported recommendation is therefore conservative: do not diagnose N supply from one pool, and do not reduce fertilizer from a global rule. Use replicated field strips to compare candidate packages. Keep residue and tillage treatments explicit. Measure crop status during the decision window and close the loop after harvest.

A SoilBeat perspective

This study fits a Measure → interpret → recommend → act → monitor approach because the agronomic signal sits between datasets. A mineral-N test alone could suggest less supply. Total soil N alone could suggest abundance. The crop response becomes clearer only when N form, residue management, tillage, crop sequence, moisture, plant analysis, yield, and quality are interpreted together.

Structured calculations can track nutrient inputs, removals, residue credits, and partial productivity. Agronomic reasoning must then account for mineralization, immobilization, root access, and sampling timing. After action, monitoring shows whether the chosen package maintained crop performance, built useful N reserves, reduced residual risk, and improved margin.

under conservation agriculture, the most important nitrogen question may not be how much N is present today, but how much is protected, how much can become available when the crop needs it, and how confidently the field measurements can distinguish the two.

References

Siddhartha Mukherjee, Dibyendu Sarkar, Sahely Kanthal and Biswapati Mandal (2026). Conservation agriculture optimizes soil nitrogen stabilization and availability to crops under rice-based cropping. Scientific Reports. Published 8 October 2026.

AI-generated editorial illustration for SoilBeat. Conceptual imagery; not a photograph or figure from the study.