Agroecosystems

The term ecosystem was first coined by a British ecologist, Sir Arthur George Tansley, to denote complex systems comprising both abiotic and biotic components. Agroecosystems, or agricultural systems, are specialized, human-managed ecosystems in which communities of plants and animals interact with their physical and chemical environments modified for the production of food, fuel, and fiber for human consumption and processing. Agroecosystems are more open than natural systems (forests).

They rely on the external inputs of energy and nutrients, and most of the production is removed out of the system. In general, agroecosystems are not self-sustaining and are driven by population, market, and policy needs and regulated by environmental feedback mechanisms. Agroecosystems are the basic unit of agroecology studies. Specifically, agroecology refers to the science of understanding the ecology of agroecosystems. However, in recent times agroecology has denoted an integrated discipline that includes elements of agronomy, ecology, sociology, and economics.

Agroecosystem dynamics can be studied using key properties such as productivity, sustainability, stability, equitability, and autonomy. These are also called system properties or emergent properties. The term productivity relates to the quantity of food, fuel, or fiber that an agroecosystem can produce for human use. The term stability refers to consistency of production, while sustainability refers to the maintenance of a specified level of production over the long term. Equitability refers to sharing agricultural production fairly, whereas autonomy refers to agroecosystem self-sufficiency. These properties can be used to compare the performance of agroecosystems in diverse landscapes and at varied spatial scales.

However, assessing agroecosystem properties is more complicated as the properties themselves have multiple dimensions and meanings. The definitions also vary based on the scale at which the properties are analyzed. For example, the productivity of a shifting cultivation field may be high per unit area of land on the cultivated field itself, but the productivity may be low in terms of the total land area occupied by the entire shifting cultivation cycle including forest fallow.

Structure and Function

Significant structural and functional differences in agroecosystems versus natural ecosystems have been noted by several researchers. The structural composition of an agroecosystem varies based on the location and scale at which it is studied. Agroecosystem structure can also change over a period of time due to management practices. The structure of agroecosystems can be perceived in terms of hierarchy theory.

Agroecosystems may be thought of as occurring in nested hierarchies, that is, agricultural farms nest in communities, which are nested in watersheds, which are further nested in larger regions, and so on, up to the globe. Each level in the nested hierarchy has its own unique properties (ecological, cultural, social, or economic features) and contributes to the nature of, and is affected by, levels above and below it. Across each level, agroecosystems may exhibit different interactions. For example, at a farm level, the input elements of agroecosystems include land, labor, and capital, which are interlinked together and interact with external attributes such as markets, policies, etc. To understand the complex interactions among different levels of agroecosystems, researchers have identified three different dimensions, that is, environmental (for example, soil and other biophysical attributes), economic (for example, market) and human dimensions (for example, rural community). The division of agroecosystems into individual dimensions such as those described above facilitates understanding in a more systematic manner.

In comparison to natural ecosystems such as forests, conventional agricultural ecosystems exhibit a simplified, trophic structure due to few selected crops or animal types. Agroecosystems consist of three intermingled and strongly interacting subsystems: the managed fields, referred to as the productive subsystem; the semi-natural or natural habitats surrounding them; and the human subsystem composed of settlements and infrastructures. Agroecosystem function is a consequence of agroecosystem structure, which can vary widely based on location and management practices. Agroecosystem functions involve movement of material, energy, and information from one agroecosystem to another or movement in and out of the agroecosystem. Compared to natural ecosystems, agroecosystems are characterized by relatively high net productivity, simple trophic chains, low genetic and species diversity, low habitat heterogeneity, etc. Further, the multiple facets of agroecosystems and the services they offer have been well recognized. Agroecosystems offer a variety of functions. The functions refer to how agroecosystems operate in generating different services, which include production services, ecosystem regulation, and cultural services. Furthermore, these ecosystem services are strongly dependent upon the management practices.

The production services of agroecosystems include food, fuel, and fiber. Agroecosystems also serve as repositories of agrobiodiversity and genetic resources. The ecosystem services include providing habitat for biodiversity, primary production, biogeochemical cycling, soil formation and retention, carbon sequestration, water cycling, energy flow, etc. Regulation services include soil erosion control, climate mitigation, water purification, pollinating mechanisms, etc. The cultural services include agroecosystems serving as knowledge systems for education and inspiration, spiritual and religious values, recreation/aesthetic importance, etc.

In contrast to the above-mentioned positive ecosystem services, agroecosystems can also impact the environment in a negative way depending on management practices such as nutrient runoff due to excess nutrient application, greenhouse gas emissions, sedimentation, pesticide poisoning of humans, etc.

Livestock

Livestock are an important component of traditional agroecosystems and are considered a secondary food production system. Livestock provide draught power for farm operations including plough operations. Livestock are also the source of organic manure for crops and the main source of protein for human beings through the supply of meat and milk. In addition, livestock provide a source of income through the sale of animals. Compared to the specialized agroecosystems, such as single-cropping production systems or livestock-alone systems, integrated crop-livestock agroecosystems are considered highly beneficial in terms of agricultural productivity, environmental quality, operational efficiency, and economic performance. The synergies brought about by integrating crop and livestock systems can result in a positive feedback. For example, the crop residues that are left after harvesting can serve as a feed, while the livestock excreta can serve as manure to soils.

Integrated crop-livestock agroecosystems have also been associated with positive environmental effects. Increased organic carbon and nitrogen in soils of crop-livestock systems due to livestock manure have been associated with greater aggregate stability of soils, thereby conferring resistance to soil erosion. Another improvement obtained by diversifying solely from cropping systems or livestock systems and choosing integrated cropping-livestock systems is mitigation of losses from disturbances in either of the individual systems. In essence, crop-livestock agroecosystems can achieve both profitability and environmental benefits.

Agroecosystem Health

The concept of agroecosystem health has been thoroughly debated in literature. Some researchers focus on symptoms of ecosystem stress, whereas others focus on ecosystem organization and change in relation to stress. In recent times, the health concept is generally judged to be well suited to describe the state of agroecosystems given the many ecosystem services that agroecosystems provide and their importance to human livelihood. In general, agroecosystem health has been closely linked to agroecosystem sustainability. Researchers preferring the agroecosystem health concept argue that sustainability is a broader concept while agroecosystem health is a more focused term. To describe agroecosystem health, different indicators have been proposed by earlier researchers, which vary from the farm to the landscape level. For example, agroecosystem health might be evaluated on the basis of structural criteria (resource availability, accessibility, diversity, equitability, and equity), functional criteria (productivity, efficiency, effectiveness), organizational criteria (integrity/coherence, self-organization, autonomy, self-dependence/self-reliance), or dynamic criteria (stability, resilience, capacity, and time to respond).

Alternatively, agroecosystem health evaluation could integrate different ecological processes such as cybernetics, water cycle, mineral cycle, and community dynamics in addition to human and biophysical phenomena and processes at different spatial scales. Several authors tried to quantify agroecosystem health, some using a conceptual framework, others using driving forces, and others by integrating biophysical and socioeconomic datasets using spatial tools and a multicriteria decision-making framework.

Scale

There is a significant need to understand scaling aspects in agroecosystems. Agroecosystems exist and function at plot level (less than a hectare) to landscape scale (many square kilometers). The information that is generated at the plot or field level cannot be generalized to the regional, national, or global level. While most of the decisions made by policy makers are at a regional or national scale, those decisions may not be congenial to implement at the plot or farm level. Thus, interdisciplinary approaches are needed to link plot level data and extrapolate it to landscape scale. To implement any such approaches, the data requirements for analyzing agroecosystem properties and functions at multiple scales can vary. Extensive georeferenced data might be needed to quantify some of the agroecosystem properties, and such data are rapidly becoming available. With advances in remote sensing technology and availability of improved satellite data, crop-type mapping, including field size, is plausible. Further, geospatial information technologies facilitate storage analysis, retrieval, and display of spatial and nonspatial information useful for agroecosystem research. Both local and federal government agencies are rapidly developing geospatial technologies to establish databases and make them available to the scientific community. Therefore, characterizing agroecosystem properties and functions at different scales will become more feasible over time.

Sustainable Agroecosystems

There is a growing recognition in the scientific community of the importance of adopting more sustainable and integrated practices for agricultural production in diverse landscapes that depend less on chemical and energy-based inputs. Relating to the same, the term sustainable agriculture is most commonly used to synthesize a variety of concepts and perspectives associated with agricultural practices that differ from conventional production. Conventional agricultural production relies heavily on external inputs. For several years the conventional practices emphasized short-term economic and production gains at the expense of long-term economic, environmental, and community interests. For improved agricultural management, the adoption of more holistic systems-based approaches, such as permaculture, have been proposed as an alternate path. The goal is to develop sustainable agroecosystems that have high production efficiency, economic viability, environmental compatibility, and social acceptability with less impact to the environment. Although designing such agroecosystems is highly challenging, efforts are underway through farming systems research.

Studies suggest organic farming as highly sustainable in the long term. Organic farming relies on management practices that enhance organic matter in the soil and limits the use of synthetic fertilizers, pesticides, plant growth regulators, livestock antibiotics, or genetically modified organisms. It also relies on improved crop rotations such as involving legumes, green manure, compost, and biological pest control. As defined by N. H. Lampkin in 1994, the aim of organic farming is “to create integrated, humane, environmentally and economically sustainable production systems, which maximize reliance on farm-derived renewable resources and the management of ecological and biological processes and interactions, so as to provide acceptable levels of crop, livestock and human nutrition, protection from pests and disease, and an appropriate return to the human and other resources.''

Though organic agriculture is closely linked to sustainability, it can also have negative impacts to the environment such as the leaching of nitrates through legumes, or volatilization of ammonia from livestock waste. However, several researchers agree that the negative impacts caused by organic farming are low compared to conventional systems.

Four different attributes are most commonly used to assess agroecosystem sustainability in farming systems. They include diversity, cycling, stability, and capacity. Maintaining diversity in agroecosystems is important for risk minimization, genetic conservation, efficient resource use, and biological pest control. Cycling refers to effective flows of nutrients. Open nutrient cycles lead to losses, whereas closed cycles ensure that nutrients remain in the system. Thus, agroecosystems having closed or tighter nutrient cycles are better performing systems and are much more sustainable. Stability is also called homeostasis or resilience by different researchers. These terms refer to low variability or resistance to change and the ability of a system to maintain its productivity when subject to disturbances. Capacity refers to quality of the soil and water resource base and their ability to produce and sustain biomass. These attributes together with surrogate variables can be used to quantify the sustainability of farming systems.

Economic development and sustainability of agroecosystems will depend largely on our ability to manage these agroecosystems in a systematic manner. Maintaining crop diversity in agroecosystems is considered one of the important factors contributing to ecological sustainability. However, crop diversity can contribute to ecological sustainability only if the different crops fill various functions (for example, nitrogen fixation, production of organic matter to maintain soil quality, and provision of ground cover to prevent erosion) necessary for maintaining a productive agroecosystem. In addition, a diverse cropping system can reinforce monetary productivity and stability by allowing farmers to adjust the areas they plant to different crops in response to market opportunities.

Agroecosystems and Role in Climate Change

Climate change is mostly attributed to the increase in greenhouse gases in the atmosphere. The changes in greenhouse gas concentrations are projected to lead to regional and global changes in climate and related parameters such as temperature, precipitation, or soil moisture. These changes can also affect agroecosystems and their productivity. Further, agroecosystems act as both sources as well as sinks for greenhouse gases.

In 2020, agriculture itself accounted for 11 percent of greenhouse gas emissions through the release of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Thus, agroecosystems have a major role to play in the mitigation of these gases. CO₂ is mostly released through deforestation for agriculture expansion. CO₂ is also released during the burning of agricultural crop waste, for example, rice or wheat residues. In Asia, agricultural residue burning is common as it is an easy way to dispose of residues. Fossil fuel combustion during agricultural management practices also releases CO₂. One of the largest sources of CH₄ from agriculture is through paddy fields. Because the paddy fields during most of their cropping period are submerged in water, CH₄ is released due to anaerobic decomposition. Other sources of CH₄ include animal husbandry (particularly of cattle and other ruminants), crop residue burning, and food waste. CH₄ persists for a shorter time in the atmosphere than CO₂ but traps at least twenty-eight times as much heat over a hundred years. Most of the N₂O emissions from agricultural systems are from nitrogen fertilizers, leguminous crops, animal waste, and agricultural residue burning. N₂O traps more than 250 times as much heat in the atmosphere as CO₂ does. Most of the above-mentioned greenhouse gas emissions can be mitigated through management practices both in the crop and livestock systems, such as breeding low-emission ruminants, changing animal diets to reduce flatulence, and shifting patterns of flooding and drying for rice paddies.

Agricultural soils can also act as a CO₂ sink mechanism. “Carbon sequestration” refers to removal of CO₂ from the atmosphere into a long-lived stable form, which can be beneath the soils. Soil carbon sequestration is considered an effective tool for mitigating CO₂. The most common management practices that are followed in the cropping systems for soil carbon sequestration include improved tillage practices, reduced cropping intensity, and incorporating organic inputs through fertilization. Tillage and soil carbon are negatively related as soil tillage accelerates organic carbon oxidation releasing high amounts of CO₂ to the atmosphere. Thus, management practices that reduce tillage result in increased soil carbon. Cropping intensity may positively enhance soil carbon as more biomass is incorporated into the soil through crop residues. Carbon sequestration can also be maximized through fertilization options such as organic compost, livestock manure, etc., and other organic amendments.

One of the most important challenges for agroecosystems will be adapting to future climate change and unfavorable weather conditions. Developing cultivars that require either longer or shorter growing seasons based on location, that can tolerate drought conditions and heat resistance, and, most important, that can produce sufficient yields for a growing global population will be the key adaptation strategy.

Greater emphasis may also be placed on the moisture-conserving managing practices such as minimum tillage, conservation tillage, or no tillage as climate change predictions in different regions suggest moisture shortage. Practices such as intercropping, multicropping, and relay cropping may be beneficial in terms of achieving overall production per unit area and soil moisture conservation. The importance of no-till agriculture is gaining significance. It is a way of growing crops without disturbing the soil through a tillage mechanism. This practice has been shown to increase the amount of water and organic matter in the soil, increase nutrient retention, and reduce soil erosion. Cover-cropping is another option that has been shown to enhance soil fertility and soil quality, suppress weeds, and help control pests and diseases. In essence, the key for the future will be to design and manage agroecosystems that have high production efficiency as well as being economically and environmentally efficient over long-term periods.

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