Section II COV: Comparative Overviews of Important Topics For Environmental Management

II

Agricultural Soils: Carbon and Nitrogen Biological Cycling

Introduction

Carbon (C) and nitrogen (N) are two of the most important elements that affect soil productivity and environmental quality.¹'¹ Carbon is found throughout nature in a wide variety of forms and particularly in soil as 1) complex organic compounds (e.g., carbohydrates—C_xH_2xO_x, lignin, etc.,) derived from living organisms; 2) carbonate minerals such as calcite (CaCO_;) and dolomite [CaMg(C0₃)₂]; and 3) carbon dioxide (C0₂) and methane (CH₄) as decomposition end products. Nitrogen is an essential element of plants, animals, and microorganisms—a part of chlorophyll, enzymes, amino acids, and proteins, which are necessary for growth and development of organisms. In typical unpolluted soil, quantity of N in organic matter and fixed as ammonium (NH₄) in clay minerals far exceeds quantities in plant-available forms of soluble nitrate (NOJ) and NH₄. Among several soils in North America, total N in lm depth of soil was 16.0 ± 6.9 Mg N ha⁴ with 13% ± 15% fixed as NH₄ in clay minerals and <1% as soluble NOJ.¹²¹

Agriculture, i.e., the growing of plants and animals for human and livestock consumption, is a widespread land usage throughout the world. Globally, agriculture occupies approximately 38% of the total land area with 1.5 billion ha in cropland and 3.4 billion ha in perennial grassland.¹⁵¹ Addition of N, phosphorus (P), potassium (K), and other nutrients to soil is often needed to satisfy the demands by high-production crops and forages. A portion of these nutrients is naturally supplied through plant residue and soil organic matter decomposition, but amendment with inorganic or organic fertilizers is often needed to achieve high production. Unfortunately, there are many pathways for nutrients to escape from the agricultural landscape into nearby streams, lakes, groundwater, and the atmosphere. Preventing these losses is one of the goals of sustainable, ecologically based approaches to agricultural production.

Carbon and Nitrogen Cycles

Both C and N are biologically fixed from inorganic atmospheric forms to organically bound plant and microbial forms. Photosynthesis converts inorganic C0₂ from the atmosphere into organic carbohydrates in plants, algae, and cyanobacteria. Biological N fixation is a unique transformation carried out by a number of bacteria, which convert N₂ gas into ammonia (NH,) for biological utilization. N-fixing bacteria are most prevalent in symbiotic relationships with plants, such as Rhizobium that forms nodules on the roots of clovers where the nitrogenase enzyme catalyzes the reaction. Fertilizer manufacturing converts N₂ gas into NH_} in a similar manner without an enzyme, but rather large quantities of energy necessary to create the pressure required for the transformation.

Under certain conditions, both inorganic C and N can be chemically fixed in the subsoil. Carbon dioxide forms carbonic acid in water, which can precipitate with the basic cations, Ca²⁺, Mg²⁺, and Na% to form pedogenic carbonates. Inorganic C is most abundant in soils of the semiarid and arid regions. Ammonium can be fixed as nonexchangeable components of the lattice structure of 2:l-type clay minerals, which are especially prevalent in the subsoil of many younger soils.

Carbon and N occur in various forms and undergo transformations from one form to another, primarily through biochemical manipulations involving enzymes.¹⁴-⁵! Enzymes are proteins, functioning to catalyze very specific reactions either 1) intracellularly within plants, microorganisms, or soil animals; or 2) extracellularly in soil solution or attached to soil colloids. Some major enzyme categories and their reactions with C and N substrates in soil are 1) hydrolases, such as amylase and cellulase, which hydrolyze various carbohydrate and macromolecular compounds; 2) oxyreductases, which catalyze various electron transfer reactions; 3) proteinases, which convert proteins to amino acids; 4) lignocellulases, which catalyze the ecologically resistant step of lignin breakdown; and 5) lyases, which form double bonds through reactions other than hydrolysis or oxidation. Two key enzymes involved in the fixation of C and N into organic forms are 1) ribulose bisphosphate carboxylase (rubisco), which is the photosynthetic enzyme catalyzing the transformation of C0₂ from the atmosphere into carbohydrates; and 2) nitrogenase, which catalyzes the biological N fixation reaction in symbiotic bacteria associated with leguminous plant roots:

Forms and fluxes of an element are commonly illustrated in a cycle following the principles of conservation of mass (i.e., elements are transferred from one molecule to another) (Figure 1). Carbon and N cycles have global dimensions with terrestrial, aquatic, and atmospheric components of major sig- nificance.^16,71 The sun initiates a chain of energy reactions, which drive elemental cycles. The elemental cycles of C and N interact closely with the water cycle, as water is a fundamental internal component of life and a major transport mechanism of nutrients.

In natural systems without significant import of N from fertilizers, the cycling of N is largely dependent upon the cycling of C. Since growth of plants is often limited in N supply due to the strong competition for N by soil microorganisms, which have a steady supply of C-rich substrates at the surface of undisturbed soil, N losses from natural systems are typically low.^[s| The need for additional N in agricultural systems can be historically derived from two major pathways: (1) high protein harvest of grain, forage, and animal products that requires supplemental N to replace the already limited N supply in natural systems (and eventual lack of recycling waste and manure by-products from harvested food products back to the land); and (2) loss of soil-surface residue cover and soil organic matter with intensive tillage that initially stimulates N release to crops, but that eventually exhausts the soil resource in its ability to supply N to crops. Loss of C-rich surface residue and soil organic matter essentially removes the C stimulus needed to conserve N in soil, thereby resulting in major losses of N from agricultural systems with time and creating a system that relies heavily on external N inputs to supply crops with not only the N removed from harvested crops but also the N lost via leaching, runoff, volatilization, and

FIGURE 1 Generalized diagram of the C and N cycles in soil.

denitrification. Estimated global production of N and P fertilizers was 100 and 41 Tg (10^I2g), respectively, in 2007,¹³¹ which compared to about 10 Tg of production for each nutrient in I960.¹⁹¹

Autotrophic fixation of atmospheric C0₂ by plants captures the energy of the sun within organic compounds via the process of photosynthesis (Figure 1):

Inorganic N is taken up by plant roots and synthesized into amino acids and proteins during plant development. Plants are eventually consumed by animals or microorganisms, transferring portions of this stored energy through biochemical processes into various cellular components. Once in soil, the C cycle is dominated by the heterotrophic process of decomposition, i.e., the breakdown of complex organic compounds into simple organic constituents. Mineralization is the complete decomposition of organic compounds into mineral constituents:

Immobilization of N occurs simultaneously with N mineralization when soil organisms require additional inorganic N to meet the high demand for new body tissue while decomposing C-rich substrates low in available N. Net N mineralization occurs when gross N mineralization exceeds that of N immobilization.

Environmental Influences on Soil Microbial Activity

Organisms predominantly responsible for decomposition oforganicmatterandassociated mineralization of C and N are soil microorganisms, composed of bacteria, actinomy-cetes, fungi, and protozoa.^110,111 Soil fauna are larger soil organisms, such as beetles and earthworms (macrofauna, >2 mm width x >10 mm length), collembolan and mites (mesofauna, 0.1-2 mm width x 0.2-10 mm length), and protozoa and nematodes (microfauna, <0.1 mm width x <0.2 mm length), that also indirectly affect C and N cycling by 1) comminuting plant residues and exposing a greater surface area to soil microorganisms; 2) transporting plant and animal residues to new locations in the soil to facilitate decomposition, interaction with soil nutrients, or isolation from environmental conditions; 3) inoculating partially digested organic substrates with specific bacteria and enzymes; and 4) altering physical characteristics of soil by creating burrows, fecal pellets, and distribution of soil particles that influence water, air, nutrient, and energy retention and transport. With suitable environmental conditions, soil microorganisms grow rapidly in response to the availability of organic substrates rich in C and N.

Soil Temperature

Temperature controls both plant and soil microbial activity, although not at the same level (Figure 2). Plant and soil microbial activity are limited by low temperature resulting in low photosynthetic potential, as well as low decomposition potential. For many plants, net photosynthetic activity is optimized between 20 and 30°C, because at higher temperatures, plant respiration consumes energy for maintenance. In many temperate soils, microbial activity is maximized between 30 and 35°C and decreases at higher temperatures. An intermediate temperature is often ideal for maximizing C retention in soil, because optimum plant activity competes well against soil microbial activity.

Soil Water Content

Diversity of soil microorganisms is greatest under aerobic conditions, where maximum energy is obtained. However, there are a number of soil bacteria that thrive under anaerobic conditions, in which alcohols, acetic acid, lactic acid, and CH₄ become C end products via fermentation and nitrate is

FIGURE 2 Typical responses of plant and soil microbial activities to temperature.

FIGURE 3 Responses of potential soil C and N mineralization to water-filled pore space in Typic Kanhapludults in Georgia, USA [air-filled pore space would be 100 - (water-filled pore space)].

Source: Franzluebbers.¹¹²!

converted to N gases (e.g., N₂, N₂0, NO) via the process of denitrification. Soil C and net N mineralization are maximized at an optimum balance between soil moisture and oxygen availability (Figure 3). Significant denitrification occurs at water-filled pore space >70%, resulting in low availability of inorganic N to plants.

Soil Texture

Soil texture can influence both the quantity of C and N accumulation in soil and their potential mineralization. Potential C mineralization is often greater in coarse-textured soils than in fine-textured soils, due to both increased microbial predation by soil fauna and greater accessibility of organic substrates in coarse-textured soils. Organic C and N can also be protected from decomposition when bound within soil aggregates. Water-stable aggregates are a coherent assemblage of primary soil particles (i.e., sand, silt, clay) cemented through natural forces and substances derived from root exudates and soil microbial activity.

Spatial Distribution of Organic Substrates

Distribution of organic substrates in soil has a major impact on potential C and N mineralization. Potential C mineralization is often several-fold greater in the rhizosphere (i.e., 0-5 mm zone surrounding roots) than in bulk soil. However, because of the high demand for N by plant roots and the stimulated soil microflora, net N mineralization is often initially lower in the rhizosphere because of immobilization of N. Keeping soil active with roots whenever conditions are conducive for plant growth will 1) keep inorganic N at low levels (as well as keep soil covered with protective plant cover to guard against soil erosion); 2) stimulate soil biological activity; and 3) create a richly diverse soil microbial community, all of which prevent nutrients from being lost from the soil.

Surface soil often contains greater quantities of organic matter than at lower depths due to surface deposition of plant residues, as well as greatest plant root activity. Surface soil usually undergoes the most extreme drying/wetting cycles and has the greatest exchange of gases, both of which contribute to enhanced soil microbial biomass and activity. Tillage of soil with traditional agriculture redistributes organic substrates uniformly within the plow layer, often resulting in immediately stimulated soil microbial activity from disruption of organic substrates protected within stable soil aggregates.

Minimum soil disturbance with conservation tillage practices can reduce oxidation of soil organic matter and preserve more C within soil, which can have implications for potentially mitigating the greenhouse effect.¹¹³¹

Stratification of soil organic matter with depth is common in natural ecosystems and in conservation agricultural systems (Figure 4). Conservation agricultural systems are defined as those that 1) minimize soil disturbance with tillage; 2) maximize soil-surface cover with continuous plant and/or residue cover; and 3) stimulate biological activity through diverse crop rotations and integrated nutrient and pest management.

Depth stratification of soil organic matter with time occurs when soils remain undisturbed from tillage (e.g., with conservation tillage and pastures) and sufficient organic materials are supplied to the soil surface (e.g., with cover crops, sod rotations, and diversified cropping systems). Depth stratification with time can be viewed as an improvement in soil quality, because several key soil functions are enhanced, including water infiltration, conservation and cycling of nutrients, and sequestration of C from the atmosphere.¹¹⁵¹ Depth stratification of soil organic C generally reduces water runoff volume and soil loss from agricultural fields. Grasslands often reduce water runoff volume and soil loss even further than with conservation-tilled cropland due to even greater accumulation of surface soil organic matter. Total runoff loss of nutrients is often lower with conservation tillage than with conventional tillage, because of a reduction in sediment-borne nutrients (Figure 5). Soluble (or dissolved) N and P in water runoff can be a threat to water quality with excessive nutrient applications from fertilizers and manures (even under conservation management), and therefore, further research is being conducted to identify ways of reducing nutrient loss.¹³¹¹

Stratification ratio of soil organic C has been proposed as an index of soil quality, because soil-surface enrichment of organic matter is important for improving water-stable aggregation, water infiltration and storage, nutrient cycling, and soil microbial biomass, activity, and diversity.¹³²¹ In a land-use survey in the southeastern United States, stratification ratio of soil organic C was related to the total stock of soil organic C in the surface 20 cm depth (Figure 6). This relationship indicates that the majority of C stored with conservation management in these Ultisols and Alfisois of the region occurred within the surface 5 cm.

FIGURE 4 Depth distribution of soil organic C under pastured grassland, conservation-tillage cropland, and conventional-tillage cropland on a Typic Kanhapludult in Georgia, USA.

Source: Schnabel et al.^|M1

FIGURE 5 Mean loss of N and P in water runoff across several water catchment studies in the USA.

Source: Data from Van Doren et al.,¹¹⁶¹ Langdale et al.,!'⁷¹ Blevins et al.,^|ls| Seta et al.,!¹⁹! Sharpley and Smith,¹²⁰¹ Shipitalo and Edwards,^{121 * * *}! Endale et al.,¹²²¹ Endale et al.,¹²³¹ Endale et al.,!²⁴¹ Ross et al.,^|2S| Rhoton et al.,^{125 *}! Rhoton et al.,!²⁶¹ Sharpley and Kleinman,¹²⁷! Truman et al.,!²⁸! Harmel et al.,l²⁹l and Bosch.!³⁰!