6.6.1 Agronomic management for increasing nitrogen use efficiency in legumes

The nitrogen recovery for most of the field crops ranges between 20% and 50%. Considering the poor efficiency of applied nitrogenous fertilizers, urea, in particular, farmers have to apply more N fertilizers in most of the crops (Fageria, 2002). Though the external N requirement of legumes is low compared to cereals or other groups of field crops, but the NUE of legumes is also low. As per the estimate, the total annual use of nitrogenous fertilizers over the globe is around 120 million metric tonnes (FAO, 2014). Nitrogen has a crucial role in various metabolic activities of plants but at the same time, various losses are taken place in the soil-plant system (Ladha et al., 2003). The losses include soil erosion and surface runoff (Fageria, 2002), ammonium volatilization (Hutchinson et al., 2003), denitrification (Bolan and Hedley, 2003), leaching (Randal et al., 2003), etc. However, soil moisture, the permeability of the soil system, soil texture, and climatic condition play a significant role in determining the extent of losses (Davis et al., 2003). Balasubramanian et al. (2002) stated that the two main principles for improving N recovery and NUE in crops include either enhancement in the beneficial use of external and native N or reduction in losses from the soil–plant system. The following management strategies may improve the overall recovery efficiency of N in crops targeting the beneficial use of nitrogen by the crops with reduced losses.

Cropping systems involving crops having differential nutrient requirements may provide an opportunity to achieve improved NUE through a better exploration of the rhizosphere. Crops having different root systems may modify the rhizosphere with its fullest use. Reduction in nitrate leaching with increased NUE can be achieved through the addition of a legume in the cropping system. Taking into account the higher nitrogen equivalent of a legume, a subsequent crop having higher NUE can be accommodated in the rotation for achieving higher N recovery. A legume-non legume rotation may explore the rhizosphere through its higher N recovery and reduced nitrate leaching. Shifting from existing cereal-based rotation toward N-intensive crops may help achieve higher NUE.

Maintaining the optimum plant population is also a crucial factor for achieving higher yields with improved nutrient use efficiency. It has been reported many times that the suboptimal plant population is one of the key contributors to the reduced yield of many field crops. Sometimes wider spacing vis-à-vis lower plant population had a direct impact on yields as well as partial factor productivity. Inadequate supplies of soil moisture as well as excess moisture supply both have an adverse effect on the overall N recovery of crops. Both nutrient and water use efficiencies were highly correlated and interdependent. With an increase in nitrogen use or water use in crops, the efficiency of water use, as well as nitrogen use, has drastically reduced. A proper combination of moisture regime and nitrogen level is very crucial for improving the overall NUE of legumes (Huang et al., 2018).

A proper nutrient scheduling in crops may exert higher NUE. In this respect, the concept of 4R-Nutrient Stewardship, namely, time, rate, method, and source help to reduce the nutrient losses on one hand, and the other hand, it matches the nutrient application with actual nutrient demands (Johnston and Bruulsema, 2014). The timely application of N based on crop growth stages, soil moisture present in the root zone, and environmental factors may bring better utilization of nutrients with improved recovery efficiency. A rate of N fertilizer application impacts the total dry matter production of crops vis-à-vis total nutrient demand. Higher application beyond its requirement triggers the losses from the soil system and thus reduces the overall NUE to an unlimited magnitude (Mitra et al., 2019). A combination of three factors—soil testing, producer’s fertilizer N experiences, and predicted nitrogen requirements are excellent management tools that could determine the optimum N rates for a particular crop (Sharma and Bali, 2018). Optimal rates of N fertilizer mainly depend on overall N demand of crops, nutrient availability from sources other than applied N, and the corresponding efficiency of fertilizer application to increase plant-available N. Amongst various methods, split application, band placement, and deep placement of N fertilizers improve the overall efficiency of applied fertilizers. Fertilizer assignment underneath the soil surface may habitually effective than its surface application. Band placement of N-fertilizer leads to a lesser N restriction and creates a suitable microenvironment for its effective use. It is as operative as deep-placement of urea super granules (USG) in sinking losses due to volatilization leads to increase crop yield. It has been reported that deep placement of USG has a greater benefit over surface split application on soils with higher cation exchange capacity and pH, modest to dense texture, and soils of low porousness and filtration rate. Though legumes require lesser N, care has to be given equally to improve NUE through the adoption of various agronomic management practices.

Integrated plant nutrient supply (IPNS), a balanced fertilization approach involving the judicious and combined use of inorganic fertilizers, manures (green, farmyard), biofertilizers, crop residues, as well as growing legumes in the existing cropping sequences, need to be prioritized. For making the agricultural system a sustainable one, IPNS is highly effective which involves monitoring all possible pathways of nutrient flows in the production system. Legume is certainly the most crucial component and emphasis has to be given on growing legume crop either as a grain crop, green manure crop, or dual-purpose crop. It was reported from a maize–legume–rice cropping system that a fertilizer schedule comprising the accumulation of mung bean stables or Sesbania ensures greater crop yield with continued soil fertility (Rahman et al., 2013). Biofertilizers formed an integral component of IPNS and it has been reported from many studies that apart from Rhizobium, other biofertilizers also had a vital role in enhancing the productivity of various legume crops. Bhabai et al. (2019) reported higher agronomic parameters in green gram supplied with biofertilizer Rhizobium + phosphate solubilizing bacteria (PSB) + vesicular-arbuscular mycorrhiza (VAM) packages with graded doses of phosphorus. PSB transforms insoluble phosphorus to its soluble form which is helpful to offset the high cost of phosphatic fertilizers and sustaining productivity in legumes grown under different agricultural systems (Zaidi et al., 2010). Biofertilizers produced from Bradyrhizobium and Streptomyces griseoflavus used in legume crops, especially soybean and mung bean significantly promoted nodulation, nitrogen fixation, and yields (Htwe et al., 2019). Certain legume crops can solubilize the unavailable phosphates through the excretion of organic acids from the roots. Besides, legumes also help in the restoration of soil fertility, and through its rotation with any nonlegume crop in the cropping systems, it reduces the incidence of pests and diseases.

The site-specific Nutrient management (SSNM) offers a tactic for enabling farm mentors to improve fertilizer recommendations guide for a particular location. The parameters desired in SSNM are generally restrained in nutrient oversight observations conducted in farmers’ fields, for this need a minimum of one crop season. There are three basic approaches for site-specific N management such as (1) N recommendation based on soil sampling, (2) N recommendation based on condition-specific N response curve, and (3) Continuous monitoring of N status by canopy reflectance of light and leaf color chart. Use of optical sensor and normalized difference vegetation index (NDVI) has to be promoted for legumes also for getting a better response to applied N. Studies on SSNM practices on groundnut revealed higher growth and yield parameters with a higher gross and net returns under application of fertilizers based on SSNM with a target yield of 2.5 t ha⁻¹ + sulfur (S) + boron (B) + Zn compared to the recommended dose (Patil et al., 2018). In groundnut based cropping systems, namely, groundnut-sunflower (Helianthus annuus) and groundnut–maize, it was seen the nutrient application based on SSNM or soil test-based recommendation brought about higher nutrient uptake, soil available nutrients, soil microbial populations, net returns well as the benefit–cost ratio (B:C) (Kumar et al., 2021).