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Vol. 13 No. 4 - October 2007

Soil Carbon in Agroecosystems: Issues and Challenges

By: K. P. Singh1, Nandita Ghoshal2

Soil organic matter - the accumulated, decaying debris of biota living on or in the soil is comprised of highly heterogeneous components, cellular structures to amorphous compounds, including recent root exudate to extremely persistent humified material.  Being the main nutrient source for plant growth, it contributes significantly to the maintenance of soil quality. Forming the major carbon reservoir of the biosphere-atmosphere system, soil organic matter is a key determinant of carbon and nutrient cycling in the biosphere. Globally, in terrestrial ecosystems maximum amount of C is stored in soil organic matter. Carbon sequestration occurs mainly through biological transfer of atmospheric CO2 into long-lived and recalcitrant pool in the soil. Processes of soil organic C sequestration include humification, aggregation and deep incorporation of C in the subsoil. In this article we briefly discuss the current understanding and research avenues related to organic matter dynamics and C sequestration in soils with special reference to agroecosystems in tropics.

The increasing anthropogenic pressure on natural terrestrial ecosystems (notably forests) through past few centuries has caused their extensive degradation and even conversion into agroecosystems. Consequently, C stored in the soil has declined drastically in cultivated soils. For instance, in the dry tropical regions of our country agricultural soils generally show less than one-half C content relative to local forest soils. Agricultural practices typically deplete soil C because harvesting removes major fraction of photosynthetically fixed C, and therefore, least amount of plant litter is returned to the soil. Soil working in agroecosystems disrupts aggregate structure (making organic matter more accessible to biological decay), mixes fresh litter into the soil, and increases erosion of C-rich surface layer.  It is believed that agroecosystems offer immense opportunities for attaining substantial increase in soil C sequestration, serving as large C sink in global climate change context.

While the basic processes of C sequestration in soils of tropical and temperate regions are similar, the sequestration rates are generally lower in the tropics due to the prevalence of high temperatures and microbial activity throughout the year. Occurrence of long growing season in tropics, which cover approximately 40% of the world land area, raises hopes of achieving huge soil C sequestration in the region by experimenting with land use alterations and applying ecologically sound management practices. These measures will include restoration of degraded soils and ecosystems, bio-fuel offset and recommended practices on croplands and grazing lands. 

Plants supply organic matter to soil through biomass production, senescence and exudation/leaching. Soil fauna and microorganisms transform and breakdown added organic material through decomposition. Most organic compounds are processed by heterotrophic microorganisms in soil that use organic C as nutrient and energy source. Accurate quantification of inputs of C into and outputs of C from soils, often difficult to measure, is essential to assess the storage and changes of soil organic matter with time. Such measurements will help unravel the mechanisms that control C storage (to optimize the return of C to the soil) and formulate robust models of soil C dynamics and turnover. Better understanding of C dynamics, which drives fluxes of other nutrients, can be useful to search ways to improve the soil environment.

Models of soil organic matter dynamics reflect the complexity of interactions existing within the soil environment and help evaluate the effects of environmental and management changes at local, regional and global scales on rates of turnover. Most models conceptualize that C resides in soils in several discrete pools showing varying rates of turnover and loss. It is commonly assumed that soil organic matter can be fractionated into a smaller labile pool and one or more larger recalcitrant pools, each decaying according to first order kinetics. Using such approaches, several soil organic matter models have been developed, such as Century (formulated by WJ Parton et al.), Roth-C (K Coleman et al.), CANDY (U Franko et al.) and DNDC (C Li et al.). These largely empirical models have generally provided good predictions of C loss in diverse environments, usually over longer time periods.  Despite limitations of less reliable short-term predictions and uncertainty of pool homogeneity and uniqueness, these models are helpful in organizing soil C information. When soil organic matter models are integrated within whole ecosystem simulations, better evaluation of ecosystem responses to environmental change can be done. Thus, it is possible to identify the strategies optimizing C sequestration through specific management of soil and vegetation.

Natural C inputs to soil in agroecosystems occurs mainly by incorporation of aboveground crop residues and by functions associated with root systems. However, the knowledge base quantifying the amount and timing of belowground C inputs is poor and the relative importance of root mortality and exudation in contributing to soil C inputs are also debated. While some studies using minirhizotrons and pulse labelling techniques indicate root senescence and mortality to be main contributors to C addition to soil, other pulse labelling studies suggest exudation and pre-root-mortality C loss to be more important. While live roots exude C compounds and release respiratory CO2 at variable rates through the growing season, root mortality builds up necromass which undergoes decomposition, releasing CO2 through microbial respiration (the major component of soil respiration) and sequestering C in long-lived organic matter. Our studies have shown that release of C and other nutrients from decomposing root necromass from one crop continues through the growing period of the succeeding crop. Crop/annual cycle based research on root-soil interactions and processes is required with a view to selecting possible measures for manipulating C inputs to the soil through the crop root system.

Soil C loss occurs through biological (soil respiration) as well as physical (leaching and soil erosion) processes. On a global basis soil respiration is believed to be the main C loss pathway. The microbial biomass that plays a major role in transforming inputs of organic matter also controls C loss. Our field measurements of soil respiration in dryland agroecosystems show wide variations due to seasons as well as types of soil amendments. Thus, apart from the controlling effect of environmental variables (principally temperature and soil moisture), the rates of soil respiration are strongly affected by various management practices. Available data suggests that soil respiration is significantly increased due to the accelerated oxidation of labile C caused by cropping operations. It is important to understand the processes controlling soil respiration rates in order to devise strategies for effective C sequestration.

Appropriate land management can contribute significantly to soil C sequestration by manipulating agroecosystems to generate greater biological inputs of C than losses. Farming practices since ancient days have improvised procedures that enhance soil fertility by increasing the input of plant materials (e.g. shifting cultivation where cultivation is alternated with forest regeneration and growth). Precise estimates of C input and loss from soil provide the capability to quantify in short terms changes in soil organic C storage resulting from a specific land use change; such critical methodology may become increasingly important in relative assessment of the different land use change options with respect to C sequestration. Quantifying the effects of management practices and their combinations on C sequestration is vital for improving the potential of farming systems to sequester C.

In agroecosystems both crop yield and soil C sequestration are generally increased by using organic farming, tillage reduction, residue management, choice of crops, efficient irrigation and pesticide use.  Our work in rice based dryland agroecosystem indicates that a judicious mix of high and low quality organic soil amendments tends to increase C sequestration as well as crop productivity compared to these amendments applied alone. Even weed biomass may substantially contribute to C sequestration.

Recent estimates made by R. Lal of Ohio State University, USA, indicate wide variations in C sequestration potential of different land use changes and recommended agricultural practices. For instance, land use change restoring degraded soils or conversion of marginal soils to restorative land use may sequester 50-300 kg C ha-1yr-1. Sequestration under different agricultural practices may approximate: Zero/reduced tillage 100-1000, use of cover crops 50-250, manuring 50-150, mixed farming 50-200, and agroforestry 100-200 kg C ha-1yr-1.

It must be emphasized that soil C studies might not strive only to maximize C pools; instead the approach should aim to establish a balance between amounts held in reserve (pool) and amounts used for microbial activity (flux). A balance between soil C pool and flux, and consequently soil C turnover, regulates ecosystem services related to nutrient supply. Maximum benefit can be derived from accumulated organic matter when it undergoes decomposition. But the benefits of decomposition (e.g. mineralization of nutrients and humification) can be maximized when the decomposition-products are released at the time of active growth and greatest nutrient demand by plants (synchronization of nutrient demand and availability). Better management of the annual pool-flux balance can be done by appropriate selection of crops, tillage, addition of residues and manures, irrigation, etc.  Such management efforts require wide variety of research inputs, both long-term and short-term goal oriented, in different cropping systems with a view to manipulating agroecosystem processes for sustained C balance in agricultural landscape.

1Professor Emeritus, Department of Botany, Banaras Hindu University, Varanasi. E-mail [email protected]

2Reader, Department of Botany, Banaras Hindu University, Varanasi. E-mail [email protected]

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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