Long-term fertilizer nitrogen management-Soil health conundrum

The mental model that fertilizer nitrogen(N)acts as a replacement for N mineralized from soil organic matter(SOM)needs to be revisited.Soil organic matter,the storehouse of N in soil,is one of the most important indicators of soil health.It supplies more N to crop plants than the current-year fertilizer N even when applied at high rates.Limited research shows that the application of fertilizer N above the optimum levels on a long-term basis deteriorates soil health by mineralizing SOM and depleting the soil N pool.Because soil N includes portions of fertilizer N applied in several previous years,year after year application of fertilizer N below the optimum levels will also lead to a gradual decline in soil N pool or soil health.To sustain high crop yield levels,fertilizer N needs to be applied on a long-term basis neither above nor below the optimum levels to ensure that soil health in terms of sustaining supply of soil N is maintained,if not enhanced.

The transformations of inherent as well as exogenous nitrogen(N)in soil are both fascinating and frustrating in their complexity.Nitrogen is exogenously added to soil through mineral fertilizers,organic manures,biological nitrogen fixation,and atmospheric N deposition,but a large amount of N is inherently present in soil-mostly in combination with soil organic matter(SOM)and a tiny fraction in soil solution.In comparison to fertilizer N application of up to 200-300 kg ha-1,a typical agricultural soil containing 0.10%-0.15%N may hold in soil N pool about 5 000 kg N ha-1in the top 30 cm layer.

From the perspective of agricultural ecosystems,healthy soils sustain high crop yield levels primarily by maintaining an adequate nutrient supply and a favorable environment for plant growth(Havlin,2020).Therefore,traditional assessment of the health of agricultural soils tends to rely upon indicators closely linked to SOM,which significantly contributes to the productivity of agroecosystems(Seufertet al.,2012;Lehmann and Kleber,2015).There exists a fundamental coupling of microbial carbon(C)and N cycling,and due to the close correlation between the mineralization of C and N,SOM acts as the largest source of N for plants.The N contained in soil N pool or SOM,therefore,acts as an important indicator of soil health in an agroecosystem.It plays a huge role in N nutrition of crop plants even when fertilizer N is applied to soil.

Yanet al.(2020)analyzed data published in 230 papers and revealed that crops derived on average 56.2%N(59.8%and 52.6%based on15N studies and zero-fertilizer control studies,respectively)from soil and the remaining less than 50%N was supplied by current-year fertilizer N application.Based on15N studies,maize,rice,and small grains(wheat,barley,sorghum,oats,and millets)derived 59%,68%,and 63% N from soil N pool,respectively.Interestingly,in 217 field experiments in which15N-labelled fertilizer was applied at rates even higher than recommended,Gardner and Drinkwater(2009)found that average N uptake by crop plants from soil N pool was as high as 60%.These studies explicitly suggest that soil N pool,if depleted or not maintained,will not be able to supply enough N for crop plants and sustain high crop yield levels.

In intensively cultivated agricultural production systems,soil N pool is maintained by a portion of N applied as fertilizer or manure,crop residue return into soil,atmospheric N deposition,and biological N fixation.Because less than half of the fertilizer N applied in the current year is assimilated by crops,a substantial portion of the remaining applied N can accumulate in soil(Krupniket al.,2004;Yanet al.,2014;Worrallet al.,2015),and once it becomes part of the large soil N pool,it can persist there for decades(Sebiloet al.,2013;Lewiset al.,2014).Thus,soil N pool is enriched by fertilizer N applications in previous years,and N contained in it becomes available for uptake by crop plants in subsequent years.It further suggests that it is important to apply an adequate amount of fertilizer N to maintain soil N pool and soil health.Therefore,in intensive agricultural systems,longterm application of fertilizer N at rates below the optimum levels will lead to gradual depletion of soil N pool and will not be able to support high yield levels.Another important implication of the interaction between fertilizer N and soil N is that not all unassimilated fertilizer N is lost from the soil-plant system.Only a portion of the annual fertilizer N input that exceeds the amount of N removed by harvested crops and becomes part of the soil N pool is vulnerable to losses to the environment.

In agricultural systems,soil health is also reflected in the ability of soil to respond to agricultural interventions and prevent processes that degrade it(Kibblewhiteet al.,2008).In the evolution of modern agriculture,tillage and irrigation were the two most important anthropogenic interventions,but since the mid-20th century,the use of mineral fertilizers to achieve food security for the burgeoning global population has led to a fundamental change in the functioning of soil as a medium for crop production.Mineral fertilizers being chemicals can potentially alter the natural functioning of soil,but many farmers have developed a tendency to apply fertilizers even more than recommended rates to avoid the risk of yield loss due to deficiency of nutrients supplied by fertilizers.Subsidized availability of fertilizers,particularly N,by governments in several countries such as India and China to boost food production has further encouraged the tendency to overfertilize the crops with N.Fertilizer N can positively or negatively affect SOM through several direct and indirect mechanisms(Mahalet al.,2019;Bijay-Singh and Sapkota,2023).When fertilizer N is applied at agronomically optimum rates,it increases net primary productivity leading to increased production of both above-and belowground plant biomass,which in turn increases SOM content(Fig.1).Increased net primary productivity due to application of fertilizer N at optimum doses reduces soil moisture and temperature,which indirectly decreases mineralization of SOM(Partonet al.,1987;Wanget al.,2006;Liet al.,2014).Application of fertilizer N as per the crop demand leaves very little inorganic N in soil because most of the applied is either assimilated by crops or becomes part of SOM(Fig.1).

Fig.1 Schematic diagram showing effect of fertilizer N application rate on crop yield,N use efficiency,residual inorganic N in soil,and long-term basis organic N in soil(modified from Poffenbarger et al.(2017)).RU=relative units.

Fertilizer N application at rates above the optimum levels does not increase crop biomass production more than that observed at the optimum N levels,but leads to the accumulation of residual inorganic N in soil(Fig.1)(Poffenbargeret al.,2017),which stimulates SOM mineralization resulting in its net loss.Increased residual inorganic N in soil accelerates SOM mineralization by supplying N for the growth of microbes involved in the mineralization process(Russellet al.,2009;Chenet al.,2014).Application of fertilizer N at excessive doses also reduces the C:N ratio of crop residues,thereby accelerating their decay.As the transformation of fertilizer N to soil N is mediated by microbial activity in soil,the amount of fertilizer N applied below the optimum doses dictates the extent of transformation(Fig.1).However,when applied at doses above the optimum levels,the size of microbial activity rather than the amount of applied N governs the transformation of fertilizer N to soil N.

Bijay-Singh (2018) has cited several researchers who observed increases in SOM by applying optimum levels of fertilizer N.Four meta-analyses based on long-term experiments with different crops and cropping systems in various regions of the world(Ladhaet al.,2011;Körschenset al.,2013;Geisseler and Scow,2014;Geisseleret al.,2017)reported average increases of 8%-13.4%in soil organic C content due to fertilizer N application compared to unfertilized controls.The extent of SOM buildup in tropical regions was higher than that in temperate regions because tropical soils inherently contain small amounts of SOM.Since tillage enhances the decomposition of organic matter in most soils,increases in SOM due to fertilizer N application at optimum rates in long-term experiments are higher than those in longterm experiments included in the meta-analyses.Huanget al.(2020)conducted a meta-analysis of experiments in which soil C was isotopically labeled and found that fertilizer N application substantially increased new soil C stocks.

Concludingly,fertilizer N management needs to evolve considering the huge role of soil N in the N nutrition of crops and to ensure that soil health in terms of sustaining the supply of soil N is maintained,if not enhanced.Application of optimum fertilizer N levels for crop production,ideally on a field-specific basis,should ensure sustainable high yields with healthy soils and minimal losses of N to the environment.Using too much or too little fertilizer N in agroecosystems on a long-term basis leads to soil health deterioration in terms of reduced buildup or accelerated mineralization of soil N,which cannot sustain high yield levels.The pursuit of high crop yield levels by applying excessive fertilizer N year after year will lead to a spiral of decline in soil functioning and crop productivity,which will necessitate the application of even more fertilizer N every year to produce the same yield level.