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Riparian ecosystems are amongst the most vulnerable ecosystems of the world. The natural gradients and increasing human perturbations under these ecosystems can be explored for understanding the soil carbon (C) dynamics, especially soil carbon dioxide (CO 2 ) efflux. However, studies on soil CO 2 efflux and its governing variables under different land uses of dry tropical riparian ecosystems are limited. Therefore, the present study aimed (1) to assess the impact of riparian land use on soil CO 2 efflux, and (2) to identify the key drivers of soil CO 2 efflux along the river Ganga, Varanasi, India. The riparian land uses taken in this study were moist sandy flat (MSF), uncultivated sandy land (USL) and cultivated sandy land (CSL) depending upon their slope and distance from river body to upland, respectively. Soil CO 2 efflux and other soil biophysical properties were measured at 54 locations distributed in six sites having these land uses, in dry season of 2014–15. Soil biophysical properties considered in this study were soil organic C, soil moisture, bulk density, porosity, fine particles, microbial biomass C and soil pH. Riparian land uses were found to have significant impact over soil CO 2 efflux with a respective increase of 222, 424 and 63%, for MSF to USL, MSF to CSL, and USL to CSL land use transitions ( P 0.01), respectively. Similarly, the regulators of soil CO 2 efflux varied with the land uses. It showed strong positive correlation with soil organic C (r = 0.81), fine particles (r = 0.64) and porosity (r = 0.61), whereas negative correlation with soil moisture (r = 0.61) and bulk density (r = 0.62) for overall dataset. However, soil organic C, fine particles, microbial biomass C and soil pH at MSF; soil organic C and microbial biomass C at USL; and soil moisture, porosity and microbial biomass C at CSL land uses were observed to regulate soil CO 2 efflux. The findings revealed that riparian land uses have significant control over soil CO 2 efflux and its biophysical regulators which have relative control over it. Soil organic C, soil moisture, fine particles, porosity and microbial biomass C were identified as prevalent regulators of soil CO 2 efflux under dry seasons. Overall, the results indicate that the biophysical variables in addition to human interferences (CSL land use) have pronounced regulation over soil CO 2 efflux in dry tropical riparian ecosystems.

The current anthropogenic activities and climate change are increasingly becoming a growing global concern for dry tropical forests. Worldwide, these ecologically important forests have degraded considerably since the past few decades due to such factors. These factors have harmful consequences on the vegetation structure and diversity especially tree seedlings, which may further aggravate climate change. Generally, the vegetation recovery is very slow and unpredictable in the dry tropics due to complex interaction among tree seedling, site (particularly, soil) and climatic conditions. We inculcated that a better understanding of the behavior of individuals of different tree species at seedling stage in dry forests is of immense importance. It is increasingly being recognized for explaining and managing the future composition of plant communities under changing environmental conditions. In this regard, the multi-factorial interaction studies under various resource–disturbance combinations are needed in dry tropical ecosystems to understand the: (1) impact of relative variability in resources and disturbances on the responses of tree seedlings of native species and (2) how the later relates to distinct functional and life history traits of the individual tree species. Most importantly, such studies would improve our limited understanding of how variation (natural and man-made) in nutrient availability, under the influence of other local environmental factors (such as water, light, grass competition, herbivory, fire, allelopathy and enhanced CO2 conditions), would affect the dynamics of dry tropical forest community. It may help in the proper management of these forests. Moreover, it may prove helpful in the current climate change scenario, as change in forest community dynamics may have consequences on soil C sequestration and CO2 efflux at global scale.

It is now evident that the soil ecosystem is at the core of agricultural, socioeconomic, and climatic problems. Our understanding of the agroecological interactions required to directly manage the soil ecosystem for sustainable benefits is limited. This is evident by the declining soil fertility and increasing erosion and cost of agro production under current agricultural management systems. The physical, chemical, and biological characteristics of soil have been studied extensively, either separately or in limited combinations, for enhance our understanding of the effects of agro-management on soil carbon dynamics and soil quality. However, it is increasingly being recognized that an integrative understanding of the interaction between soil’s physical, chemical, and biological attributes is required for sustainable soil management in a cost-effective manner. Therefore variables integrating the spatiotemporal dynamics and microscale interactions can be used as a surrogate to understand and, thus, indirectly manage such belowground interactions. Soil aggregate dynamics might be one such integrative functional trait, which has largely been ignored in the sustainable management of agro-ecosystem soils despite having a long history of over a century. This chapter advocates for the indirect management of belowground interactions in soil at macro-scale via assessment of soil aggregate qualitative (fractionation) and/or quantitative (carbon pools and turnover) characteristics in a site-specific manner, which may provide a cost-effective and promising strategy for sustainable soil management. Therefore, we aim to explore this downplayed variable for its potential use as a surrogate to indirectly manage belowground interactions. We observed that emerging management practices such as organic amendments-based agriculture showed a beneficial role in nutrient dynamics, in contrast to inorganic fertilization which needs to be explored in relation to the aggregate dynamics for the holistic management of the soil ecosystem.

The changing environmental conditions, especially global warming and climate variability, are major concerns and have an adverse impact over the future of agriculture. The changing climate has become a global issue which needs rapid policy development at national and international levels. The melting of massive Himalayan glaciers would considerably change the impact of temperature regimes which is necessary for blossoming of several crop plants. Thus, the current generation needs to find solutions to combat the negative impacts of environmental consequences on agricultural crops. Useful strategies include the conservation of the world’s huge biodiversity together with other natural resources. Modifications in currently used agricultural practices and the search for new plant species tolerant to different abiotic and biotic environmental stresses could help to mitigate the negative impact of climate change.

Organic amendment in agro-ecosystems positively affects the soil organic C (SOC) dynamics, both alone and in combination with mineral fertilizers. Therefore, it is being widely advocated in recent times for the restoration of SOC. Limited understanding exists on how these agro-management practices affect SOC dynamics in the dry tropical agro-ecosystems. We, therefore, aim to investigate the impact of sole organic and combined (organic + inorganic) amendment on SOC dynamics, taking the precursor native forest as a reference. SOC, soil CO 2 efflux, soil physico-chemical, microbial and aggregate characteristics were observed in these systems. Our results demonstrate that organic and combined amended systems behaved distinctly as compared to reference system. However, combined amended system was found more C protective as compared to organic amended system. Further, combined amended system showed higher SOC, soil bulk density, NH 4 + -N, NH 4 + -N to NO 3 − -N ratio, microbial biomass C/N ratio (MBC/MBN), β-glucosidase activity, mineral fractions, and aggregate-associated total C as compared to organic system. However, soil CO 2 efflux, pH, moisture content, NO 3 − -N, MBC, MBN, percent labile C in meso- and micro-aggregate, and labile C stock in micro-aggregate showed an opposite trend. Soil moisture and macro aggregate water stability were found important drivers of SOC dynamics in dry tropical ecosystems. SOC dynamics across the studied agroecosystems was found associated with the shift in MBC, MBC/MBN and β-glucosidase activity. The consequent shift in the relative availability of soil inorganic-N pools (i.e. soil NH 4 + -N to NO 3 − -N ratio) was identified crucial in this regard. Its relationship with soil microbial biomass, in addition to bulk density, aggregate size distribution and labile C concentration and stock in meso- and micro-aggregates depicts its possible importance in SOC dynamics. Results suggest that higher soil NH 4 + -N to NO 3 − -N ratio in combined amended system may be responsible for its relatively higher C protective nature. It might be attributed to its relationship with labile C in meso- and micro-aggregate fractions; however, micro-aggregate appears to be relatively more important in this regard.

The dry tropical forests are among the most vulnerable ecosystems of the world and, however, are relatively understudied. These forests provide various ecosystem services, and are progressively being converted into patches of dry scrubs, savanna and marginal cropland systems, due to various anthropogenic perturbations. Soils of these regions are relatively nutrient poor with a patchy nutrient and water distribution pattern. Therefore, the variability in these natural resources imposed by the present climate change scenario may affect the forest plant community of dry tropics via its impact on seedling growth and recruitment. Seedlings are considered as the most sensitive stage of plant lifecycle, and therefore, understanding of seedling regeneration may help in restoration of forest ecosystems. Seedling growth is majorly regulated by various naturally occurring resources (such as light, water, nutrient, etc.) and disturbances (such as defoliation, grass competition, fire, etc.). Therefore, efforts on the regeneration of these forest systems are highly necessitated. In the present study, we critically reviewed the studies on seedling survival and growth under different resource and disturbance regimes with a special focus to dry tropical environment. We found that water, light, nutrients, herbivory, and grass competition majorly regulates recruitments, growth, and establishment of the tree seedling in dry tropical environment. Most of the studies are limited to observe the effect of one or two factors over the seedling survival and growth. However, the resources and disturbances may have an interactive effect over seedling growth. Therefore, studies encompassing the interactions of various growth factors (resources and disturbances) under different climatic conditions are urgently needed for the successful regeneration of tree seedlings and for the restoration of plant community. Moreover, it will improve our ability to manage the tropical vegetation under changing climatic scenario.

Land use change (LUC) may detrimentally affect the soil organic carbon (SOC) within different soil fractions; directly supplemented by significant contribution to soil CO2 efflux (SCE). To understand the governing mechanism, experimental data were collected for SOC and SCE along with soil physico-chemical, microbial and aggregate characteristics across adjacent secondary forest (SF)-grassland (GL)-cropland (CL) sequence in dry tropical ecosystems. A significant change in SOC and SCE was observed from SF to GL and CL systems, respectively; though moderately from GL to CL system. Respective decrease in SOC (31 and 42%); soil ammonium-N to nitrate-N ratio (ANR; 96 and 86%), microbial biomass C (MBC; 30 and 50%), nitrogen (MBN; 6 and 33%) and MBC/MBN ratio (25 and 24%); whereas increase in SCE (43 and 57%) and soil nitrate-N availability (340 and 592%) was observed from SF to GL and CL systems. Moreover, aggregate physical distribution shifted toward smaller size fractions; whereas aggregate-associated total C and KMnO4-labile-C concentration and carbon management index (CMI) across aggregate-size fractions decreased linearly with the land use sequence. SOC was majorly governed by macro-aggregate water stability (WASmacro) and MBC; whereas SCE by CMI of macro-aggregate (CMImacro) fraction. Furthermore, the ANR showed positive correlation with microbial (i.e. MBC and MBC/MBN ratio) and macro-aggregate physical (i.e. WASmacro) and chemical stability (i.e. CMImacro). It indicates that a shift in the microbial community with the land use may affect the relative availability of inorganic N pools and associated aggregate characteristics. Thus, our results indicate that a shift in ANR with LUC may be an unexplored and crucial indicator of soil C dynamics mediating quantitative and qualitative changes in microbial and aggregate characteristics in dry tropical ecosystems. Further, a critical emphasis is needed on the relationship of SOC dynamics with ANR for future studies at various spatiotemporal scales worldwide to recognize its potential role as ecological indicator of SOC dynamics. Also, its inclusion under climatic models may help to better predict the future climate.

The holistic understanding of soil C dynamics with age of organic amendment is limited in dry tropical ecosystems, which may act as potential C sink if managed properly. Therefore, we studied soil physicochemical, microbial, aggregate and available C characteristic with age organic amendment across the season for mechanistic understanding of soil C accumulation. Organic systems under 1, 3, 5 and 10 years under organic amendment were selected along with a forest system as reference for this purpose. We observed that soil organic C and CO2 efflux (SCE) increased significantly after ten years of organic amendment (though with no change in SCE on unit C basis), and approached closer to reference forest site. Soil fine particles, moisture content, ammonium-N, ammonium-N/nitrate-N ratio (ANR), microbial biomass C, N and C/N, crude dehydrogenase, glucosidase and phosphomonoesterase activity, macro- and mesoaggregate, mean weight diameter, macroaggregate water stability, and total and labile C concentration and carbon management index across aggregate fractions significantly increased with age. However, soil nitrate-N, C/N ratio, water soluble C, specific glucosidase and phosphomonoesterase activity, microaggregate and mineral fraction decreased. The seasonality of total and labile C concentration and stock differed distinctly across aggregate size fraction, which showed its linkage with microbial and plant C input dynamics. Though, total and labile C stock increased across aggregate fractions with age, it shifted towards macro-and mesoaggregate, opposite to smaller size fractions. Macro-aggregate stability along with macro-aggregate labile C, dehydrogenase activity, microbial biomass C/N and mesoaggregate C majorly determined the SOC. However, specific glucosidase activity along with mesoaggregate, ammonium/nitrate ratio and microbial biomass majorly governed the SCE. The study indicates that macro-aggregate characteristic (including mesoaggregate) play a crucial role in soil C accumulation in dry tropical ecosystem under organic amendment, with microbial community characteristics and ANR. Moreover, the study indicates towards key role of macro-mesoaggregate equilibrium and inorganic N dynamics in the soil C dynamics of the studied ecosystems, in which seasonality and C input play a crucial role.

Plasticity in physiological traits is necessary for the survival and development of woody species in the severe conditions of tropical dry forest We selected five study sites in a gradient of soil moisture availability located in dry forest of India We identified physiological traits viz relative water content RWC leaf dry matter content LDMC specific leaf area SLA leaf carbon concentration LCC leaf nitrogen concentration LNC leaf phosphorus concentration LPC chlorophyll concentration Chl stomatal conductance Gsmax photosynthetic rate Amax intrinsic water use efficiency WUEi biomass increment Bio Incr relative growth rate RGR which are considered important for the survival and growth of plant species in tropical dry forest and measured their range and plasticity in woody species including trees and shrubs across the selected study sites Further we analysed the response of physiological traits to variations in soil moisture content SMC across species as well as across study sites Across the five study sites the selected traits exhibited remarkable plasticity both within as well as among species The associations of physiological traits with soil properties were also significant The study shows that all physiological traits under study affect RGR directly or indirectly However the strength of effect is determined by environmental parameters particularly the SMC Step wise multiple regression indicates that more than variability in RGR can be explained by SLA and WUEi alone We suggest that for predicting the vulnerability of tropical dry forest communities to changes in climatic conditions further investigations examining trade offs among physiological traits and habitat conditions are needed nbsp nbsp

Globally, land use change and management have declined soil organic carbon (SOC), thus emitting more CO2 contributing to global warming. Here we review factors that control the fate of soil organic carbon. We found that dry tropical soils are considerably away from carbon saturation, and thus have the potential for high carbon sequestration, if managed properly. Integrated indicators have been set up, such as relative availability of inorganic nitrogen pools, carbon management index, macro-aggregate water stability and metabolic quotient. For example, the relative, rather than absolute, availability of inorganic nitrogen pools has been found associated with resource conservation mechanisms in soils.

In the extremely variable and severe environment of tropical dry forest the phenotypic plasticity of morphological traits of plant species plays an important role for their development functioning and evolution We selected eight morphological traits viz girth GT height HT bark thickness BT wood specific gravity WSG leaf area LA crown cover CC crown depth CD leaf area index LAI which are considered important for the survival and growth of plant species in tropical dry forest and measured their range in woody species including trees and shrubs across the five study sites We also measured phenotypic plasticity of morphological trait of each woody species across the study sites In our study the woody species exhibited wide range of morphological traits The phenotypic plasticity of the morphological traits showed remarkable variation across species as well as within a particular species growing at different at different levels of soil moisture content The relationships of mean value of morphological traits across all woody species at five study sites with the respective mean value of soil properties were significant Among the selected traits LA and HT were most important for the variation in SR at our study sites We suggest extensive investigation of the phenotypic plasticity of the morphological traits of woody species in tropical dry forest for proper management and sustainable development of the forest ecosystem