Labile soil organic matter pools (LSOMp) are believed to be the most sensitive indicator of soil quality when it is changed rapidly with varied management practices. In sub-tropical climates, the turnover period of labile pools is quicker than in temperate climates. Organic amendments are of importance in improve the LSOMp for a temperate climate and may be helpful in sub-tropical climates as well. Hence, the status of LSOMp was studied in long term farmyard manure (FYM) amended soils under wheat (Triticum aestivum L.) and pearl millet (Pennisetum glaucum L.) cropping systems in sub-tropical arid conditions. At the same time, we also attempt to determine the impact of mineral nitrogen (N) application in these pools. In this study, dissolved organic matter (DOM), microbial biomass (MB), and light fraction (LF) were isolated in the management practices involving different modes and rates of FYM applications along with the application of nitrogenous fertilizer. C and N contents of the labile pools were analyzed in the soil samples at different periods after FYM applications. Among the different pools, microbial biomass carbon (MBC) and dissolved organic carbon (DOC) were changed significantly with different rates and modes of FYM application and mineral N application. Application of FYM at 15 Mg ha⁻¹ in both the seasons + 120 kg ha⁻¹ mineral N resulted in significantly higher MBC and DOC as compared to all of the other treatments. This treatment also resulted in 13.75% and 5.8% more MBC and DOC, respectively, as compared to the amount of MBC and DOC content in the control plot where FYM and mineral N were not applied. Comparing the labile organic matter pools of 45 years of FYM amendment with initial values, it was found that the dissolved organic carbon, microbial biomass carbon, and light fraction carbon were increased up to the maximum extent of about 600, 1200, and 700 times, respectively. The maximum amount of DOM (562 mg kg⁻¹ of DOC and 70.1 mg kg⁻¹ of DON), MB (999 mg kg⁻¹ of MBC and 158.4 mg kg⁻¹ of MBN), LF (2.61 g kg⁻¹ of LFC and 154.6 g kg⁻¹ of LFN) were found in case of both season applied FYM as compared to either summer or winter applied FYM. Concerning the different rates of FYM application, 15 Mg ha⁻¹ FYM also resulted in a significantly higher amount of DOM, MB, and LF as compared to other FYM rates (i.e., 5 Mg ha⁻¹ and 10 Mg ha⁻¹). Amongst different pools, MB was found to be the most sensitive to management practices in this study. From this study, it was found that the long-term FYM amendment in sub-tropical soil along with mineral N application can improve the LSOMp of the soil. Thus, it can be recommended that the application of FYM at 15 Mg ha⁻¹ in summer and winter with +120 kg ha⁻¹ mineral N can improve SOC and its labile pools in subtropical arid soils. Future studies on LSOMp can be carried out by considering different cropping systems of subtropical climate.

Rice–wheat cropping system (RWCS) is considered as the furthermost vital system in the Indo-Gangetic Plains of South-Asia, including India, Pakistan, Bangladesh, and Nepal. Recently, the deficiency of micronutrients like zinc (Zn) has emerged as one of the prime limitations for the sustainability issues of this RWCS in Zn deficient calcareous soils, particularly in India, as a result of the calcareous typic ustifluvents taxonomic nature of the soils. Therefore, a new Zn fertilization approach for soils is very much needed in the intensive RWCS. Thus, a six-year-long investigation was designed with three different modes of Zn application, viz., the application of Zn only in the first year of study, application in alternative years, and application in every year. Four different rates of Zn applications in a hectare of area for a single year, viz., 2.5, 5.0, 7.5, and 10 kg ha⁻¹ year⁻¹, and times of Zn application, viz., only at first year, alternative years, and in each year. The major aims of the study were to determine Zn concentration in soil; yield; and Zn accumulation by wheat crop under different application methods of Zn. From this study, it was found that a large portion of the applied Zn was present in the residual fraction, and it was also revealed that increasing the frequency of Zn application resulted in the increment in the crystalline fraction. Significant correlation of water-soluble and exchangeable Zn (WS+EX-Zn), complexed Zn (COM-Zn), amorphous Zn (AMO-Zn), organic Zn (ORG-Zn), total Zn (TOT-Zn), grain yield, and grain Zn uptake by wheat indicated that these Zn fractions were dominant forms in the soil to be utilized by plants under rice–wheat rotation. Concerning yield and Zn uptake by wheat, it was noted that the Zn application at 10 kg ha⁻¹ in alternate years was the best Zn application method, while application of Zn at 7.5 kg ha⁻¹ in each year also resulted in comparable yield.

Calcareous soils are highly deficient in boron (B) due to having high levels of free CaCO₃ and low organic matter. This has become one of the most important deficient micronutrients in Indian soil after zinc (Zn). For various rice (Oryza sativa L.)-based cropping systems, B fertilization is essential for increasing crop productivity and the biofortification of the crop, thus a suitable soil application protocol for B fertilization is required for B-deficient soils. In a six-year experiment, different rates of B application, namely, 0.5, 1.0, 1.5, and 2.0 kg ha⁻¹ y⁻¹, were evaluated to determine the effects of three different modes of B fertilization, i.e., applied only in the first year, applied in alternating years, and applied every year, in a rice (Oryza sativa L.)–Indian mustard (Brassica juncea L.) cropping system. It was observed that the application of B at 1.5 kg ha⁻¹ every year or 2 kg ha⁻¹ in alternate years resulted in the highest yield of rice and mustard, as well as the maximum system productivity of the rice–mustard cropping system. Application of 2 kg ha⁻¹ of B in the initial year showed the maximum B uptake by rice, while application of 1.5–2.0 kg ha⁻¹ of B every year resulted in the maximum B uptake by the mustard crop. Application of B at 2 kg ha⁻¹ in alternate years or 1.5 kg ha⁻¹ every year was the best B-application protocol in B-deficient calcareous soils for ensuring the highest productivity of the rice–mustard cropping system and B availability in the soil.

Crop residue, a readily available biomass, is the largest source of organic matter in soil, and zinc (Zn) significantly influences microbial activity. Understanding the optimal Zn rates for enhanced biological activity in crop residue-amended soils is crucial. A study at RPCAU, Pusa, examined the combined effects of Zn applications and long-term crop residue amendments on soil biological properties in a rice–wheat cropping system. Conducted on Zn-deficient calcareous soil, the experiment used a split-plot design with four crop residue levels (0, 25, 50, and 100%) and four Zn rates (0, 2.5, 5, and 10 kg ha⁻¹). Crop residues were incorporated each season, while Zn was applied initially in 1994 and again in 2018. The results showed significant improvements in soil organic carbon, organic C-stock, and reductions in soil bulk density. A linear–plateau regression model revealed that Zn application at 10 kg ha⁻¹ increased soil active carbon and soil respiration by 35% and 53%, respectively, with the required crop residue levels at 73.73% and 90.28%. ACE protein increased by 9.6% with Zn application at 5 kg ha⁻¹, with a required crop residue level of 91.06%. The highest values of soil available nutrients and grain yield of rice were observed with 100% residue incorporation and 10 kg ha⁻¹ Zn application. Thus, applying 10 kg ha⁻¹ Zn along with 100% crop residue incorporation significantly improves soil biological properties and soil organic carbon levels in calcareous soil under a rice–wheat cropping system.

Zinc (Zn) fertilization is important for Zn crop biofortification as well as increasing yields, thus proper Zn recommendations for soil application is needed for Zn deficient soils. The effectiveness of Zn applications was evaluated in different combinations of rates (2.5, 5.0, 7.5, and 10.0 kg ha⁻¹ per year) and frequencies (initial, alternate, and every year) in rice (Oriza sativa L.) – wheat (Triticum aestivum L.) cropping system in a Zn-deficient upland calcareous soil in the fourth year. Zn applications to rice at 7.5 and 10 kg ha⁻¹ of alternate year and 5.0 to 10 kg ha⁻¹ of every year had the highest rice equivalent yield as compared to no-Zn treatment. Hence, Zn application to rice at 7.5 kg ha⁻¹ at alternate years is the lowest rate at which highest rice equivalent yield of rice-wheat cropping system can be obtained.