4. Discussions
4.1 Changes in SOC following LU
The exact quantification of SOC is very important for detecting and predicting the changes in response to altering global climate (Negi and Gupta, 2010). The present study showed that FL had a negative effect on SOC over other LU (BL/CL/GL/HL/PL), and the findings of meta-analysis, which is similar to the present information that LUC from FL to other LU may lead to decrease in SOC (Kolambukattu et al ., 2013). However, these results were not the universal condition for all the LUC because this test of heterogeneity showed noteworthy variability among studies. Large annual addition of OM in the form of leaf litter, SOC is highest in the FL, which exists in the soil due to the non-disturbance of any tillage activities and also slow decomposition rate due to prevailing low temperature in the forest resulted higher soil carbon values (Haynes, 2005; Baker et al ., 2007; Kolambukattu et al ., 2013) whereas in other LU the SOC was lower as when compared to FL is due to interference of human activities or tillage practices and higher temperature in these LU due to direct exposure to radiation increases the rate of mineralization resulted higher decomposition of organic matter. The decomposed organic matter releases CO2 in the atmosphere and causes loss in SOC (Ramzan et al ., 2019). Similarly in case of CL, scarce of OM addition, removal of crop residues and rigorous cultivation activities increase C losses from the soil system (Lal and Kimble, 1997; Yang et al ., 2004; Baker et al ., 2007; Smith, 2008 and Sharma et al ., 2014). The lost SOC increased the CO2concentration of atmosphere which further resulted in global warming and climate change.
To manage climate change, it is essential to store SOC in the terrestrial ecosystem. In the terms of quantity, the change in SOC content due to LUC from FL to other LU if tried to regain can be helpful to sequester more C in the terrestrial system. The study shows that the scope of improvement of SOC in other LU (BL/CL/GL/PL/HL) to become C equivalent to FL can be possible by increasing SOC by 27.3% in BL, 31.1% in CL, 36.1% in GL, 35.5% in PL and 11.5% in HL as when compared to FL. This change can be possible through proper management strategy in other LU.
4.2 Changes in SCS following LU
LUC is one of the influential factors which bring changes in SCS build up. Several studies suggest that the conversion of LU from FL to other LU led to decrease in the soil quality owing to the reduced SCS. LUC from FL into other LU may act as a C source (De Blécourt et al ., 2013; Guillaume et al ., 2015; Fan et al ., 2016; Iqbal and Tiwari, 2016) and concurrently affect soil characteristics (Abera and Wolde-Meskel, 2013). Based on study about 350 m.ha. of FL has been converted to other LU (ITTO, 2002) and leads to biodiversity loss (Ahrends et al ., 2015) and SCSt losses of 20–40%(De Blécourtet al ., 2013 and Guillaume et al ., 2015).A Meta-analysis reported that SCS decreased 13% to 42% when native FL was converted to PL and CL, respectively (Guo and Gifford, 2002). Similarly in this study the negative effect of SCS on other LU was observed in comparison to FL. The lower SCS for PL and HL was recorded in this study as compare to FL might be because of the absence of deep rooted trees and fewer canopy covers. The potential for soils to restore C from the atmosphere is affected by the balance between rate of deposition of the photosynthetic materials and rate of respiration to decompose the C inputs (Mathieuet al ., 2015). In addition, as root tissue is recalcitrant for degradation and mineralization than the top soil litters therefore root derived C has long residence time (Rasse et al ., 2005). More accumulation of organic matter can reduce the leaching (Kenye et al ., 2019). Also, absence of canopy covers, the exposure of direct solar radiation thereby increases the rate of mineralization.
The present study shows that the scope of improvement of SCS in other LU (BL/CL/GL/PL/HL) to become C equivalent to FL can be possible by increasing SCS by 34.0% in BL, 41.2% in CL, 1.47% in GL, 33.5% in HL and 47.9% in PL as when compared to FL. The changes of SCS can be supported by the study depth-wise on SCS. However this change is more in depths as compared to surface soil is due to LUC leads to difference in SCS is more at subsurface soil over the surface soil. Regaining the lost SCS amount under different LU is a tedious work. If this can become possible through management practices, a large amount of atmospheric CO2 can be restored in the soil and problems of climate change can be mitigated. Integration of organics with chemical fertilizer in cultivated soil can be one of the better LUM strategies for restoring C in the soil and improving the crop productivity and thus managing soil health and ensuring food security.
4.3. Improving SOC through nutrient management strategy
Intensive monocropping system, fertilizer-responsive varieties and continuous imbalanced chemical fertilization have caused losses in SOC (Singh et al. , 1999) and soil health (Anwar et al. , 2005; Kumar et al ., 2017 and Kumar et al. , 2018), that leads to unsustainable crop production. Organic matter (OM) addition usually acts as a “revolving nutrient fund” and as an agent to improve soil structure, maintain tilth and minimize erosion (www.fao.org) and thus converts the unsustainable to sustainable crop production system. The OM addition rapidly gaining favor but, owing to the problems related to the lack of supply of a good quality and quantity of the added materials, the system may not be sufficient to achieve and sustain the crop production for food security (Kumar et al ., 2018). Therefore, it is essential to maintain quality OM for good produce.
Balanced and INM can maintain OM levels in the soil have been well established. The INM system improves soil health through building up SOC and improving soil properties. Integrating organic and inorganic sources can help in the sustenance of farming for a longer period than chemical fertilizer alone (Sharma et al ., 2019).The present study observed that in case of INM over the inorganic alone, as the SOC status increases in the soil it helps to improve the yield of rice and wheat crop. The finding directs that SOC improves the soil physical, chemical and biological quality that provides suitable conditions for crop growth and result into good crop. Rice and wheat are the major staple food of India and main source of energy. Achieving its optimum yield level based on soil, climate and atmospheric condition will help to meet out the food security for ever growing population of India.
4.4. Effects of SOC on soil health and food security
Greenhouse gases (GHG) are the main player to maintain the global energy balance. A small change in their amount in the atmosphere can affect the climatic conditions on earth. Anthropogenic emissions of CO2 are likely to increase as in the future human population increases. Human led LUC can result into significant exchanges of CO2 between the soil and air (Lal, 2000).Therefore, restoration of C in the soil can only be option to counteract the effect of climate change and the problems created due to climate change. Since OM plays a multifaceted role in several soil processes (Gregorich et al ., 1994) hence, SOC is the panacea for sustaining soil health and food security by the maintenance of production system (Anantha et al ., 2018).
The findings suggest SOC is related to different soil properties like BD, CEC and Soil pH. Similar strong correlations has been reported in several studies of different aspects (Pal and Pawar, 2013; Wang et al ., 2014; Xu et al ., 2019). The result supports that LUC affect not only SOC but also other soil properties. Some studies have derived correlations between SOC and total nitrogen (TN) and other parameters suggesting OM turnover influences these factors (Xu et al ., 2019).
Increasing attention is being focused on the role of managing and carbon storage in the soil. However, it is a major challenge to maintain or improve low level of SCS due to LUC. Proper management strategies can be utilized to mitigate this problem.