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Nutrient budgeting for cropland is a crucial tool for assessing nutrient mining or excess application. We estimated the nutrient budget of nitrogen (N), phosphorus (P), and potassium (K) in cropland for South Asia during the last five decades (from 1970 to 2018) using equation-based empirical methods. Nutrient budget for the last five decades shows a negative balance of N (3.94 million tons, Mt), P (23.87 Mt), and K (247.23 Mt). Inorganic fertilizer remained the major input source for N and P, and its decadal average share increased for N (from 27.9% to 72.8%) and P (from 72.1% to 94.5%) from 1970 to 2010s and the share of manure, deposition, and crop residue to N, P and K input decreased. Deposition remained a major source of K input and its share decreased from 64.0% to 35.5% during the period. The share of crop removal to the decadal output of N (58.6% to 53.4%) and P (49.0% to 23.1%) decreased, and K (72.5% to 76.0%) increased from 1970 to 2010s. The higher losses of fertilizer N, and accumulation of P and K fertilizers in soils, resulted in decreasing partial factor productivity of N (from 72.2% to 16.9%), P (from 217.0% to 42.2%), and K (from 480.3% to 113.8%) from 1970 to 2018. Nutrient budget helps in identifying the regional imbalance (mining/accumulation) of the major nutrients, it will provide valuable information on the present status of country-level nutrient use for reorientation of their nutrient/fertilizer use policies.

Inland water bodies (particularly ponds) emit a significant amount of greenhouse gases (GHGs), particularly methane (CH4), carbon dioxide (CO2), and a comparatively low amount of nitrous oxide (N2O) to the atmosphere. In recent decades, ponds (<10,000 m2) probably account for about 1/3rd of the global lake perimeter and are considered a hotspot of GHG emissions. High nutrients and waterlogged conditions provide an ideal environment for CH4 production and emission. The rate of emissions differs according to climatic regions and is influenced by several biotic and abiotic factors, such as temperature, nutrients (C, N, & P), pH, dissolved oxygen, sediments, water depth, etc. Moreover, micro and macro planktons play a significant role in CO2 and CH4 emissions from ponds systems. Generally, in freshwater bodies, the produced N2O diffuses in the water and is converted into N2 gas through different biological processes. There are several other factors and mechanisms which significantly affect the CH4 and CO2 emission rate from ponds and need a comprehensive evaluation. This study aims to develop a decisive understanding of GHG emissions mechanisms, processes, and methods of measurement from ponds. Key factors affecting the emissions rate will also be discussed. This review will be highly useful for the environmentalists, policymakers, and water resources planners and managers to take suitable mitigation measures in advance so that the climatic impact could be reduced in the future.

Chemical fertilizer has contributed significantly in increasing food grain production in India. However, there are emerging concerns of environmental pollution at local scale, climate change at global scale, and sustainability of chemical fertilizer-dependent agriculture. Budgeting of nutrient is a valuable tool in assessing the nutrient use efficiency, nutrient mining, and environmental pollution. We constructed a field level top-down nutrient budget for food grain production in India since the onset of the Green Revolution in the country, i.e., 1970 to 2018, using equation-based empirical methods. Total nutrient input to Indian agriculture was 666.4 million tons (Mt) of N, 189.1 Mt of P, and 244.8 Mt of K during 1970–2018. Chemical fertilizer contributed 68.1% of N, 91.3% of P, and 28.8% of K towards the inputs. Nutrient budget for the last 48 years showed that there was positive balance of N (12.2 Mt), accumulation of P (11.7 Mt) but negative balance for K (157.9 Mt). Further, with the business-as-usual scenario, there would be positive balance of 276.2 Mt N, accumulation of 20.9 Mt P, and negative balance of 202 Mt K from Indian agriculture soils by 2050. The nutrient budget provides valuable information on the present status and balance of nutrient use and the trends with time, which will be helpful for reorienting the fertilizer use policies for sustainable agriculture. Graphical abstract

Salt-affected soils contain high levels of soluble salts (saline soil) and exchangeable sodium (alkali soil). Globally, about 932 million ha (Mha), including 831 Mha of agricultural land, is salt-affected. Salinity and sodicity adversely affect soil microbial diversity and enzymatic activities, and thereby carbon and nitrogen dynamics and greenhouse gas (GHG) emissions from soils. In this review article, we synthesize published information to understand the impact of salinity and sodicity on GHG production and emissions from salt-affected soils, and how various reclamation amendments (gypsum, phosphogypsum, organic manure, biochar, etc.) affect GHG emissions from reclaimed soils. Nitrous oxide (N2O) and methane (CH4) emissions are of greater concern due to their 298 and 28 times higher global warming potential, respectively, compared to carbon dioxide (CO2), on a 100-year time scale. Therefore, CO2 emissions are given negligible/smaller significance compared to the other two. Generally, nitrous oxide (N2O) emissions are higher at lower salinity and reduced at higher salinity mainly due to: (a) higher ammonification and lower nitrification resulting in a reduced substrate for denitrification; (b) reduced diversity of denitrifying bacteria lowered down microbial-mediated denitrification process; and (c) dissimilatory nitrate reduction to ammonium (DNRA), and denitrification processes compete with each other for common substrate/nitrate. Overall, methane (CH4) emissions from normal soils are higher than those of salt-affected soils. High salinity suppresses the activity of both methanogens (CH4 production) and methanotrophs (CH4 consumption). However, it imposes more inhibitory effects on methanogens than methanotrophs, resulting in lower CH4 production and subsequent emissions from these soils. Therefore, reclamation of these soils may enhance N2O and CH4 emissions. However, gypsum is the best reclamation agent, which significantly mitigates CH4 emissions from paddy cultivation in both sodic and non-sodic soils, and mitigation is higher at the higher rate of its application. Gypsum amendment increases sulfate ion concentrations and reduces CH4 emissions mainly due to the inhibition of the methanogenesis by the sulfate reductase bacteria and the enhancement of soil redox potential. Biochar is also good among the organic amendments mitigating both CH4 and N2O emission from salt-affected soils. The application of fresh organic matter and FYM enhance GHG emissions for these soils. This review suggests the need for systematic investigations for studying the impacts of various amendments and reclamation technologies on GHG emissions in order to develop low carbon emission technologies for salt-affected soil reclamation that can enhance the carbon sequestration potential of these soils.