May 24, 2010
Soils are the main terrestrial reservoir of nutrients, such as nitrogen and phosphorus, and of organic carbon. Synthesizing earlier studies, we find that the mobilization and deposition of agricultural soils can significantly alter nutrient and carbon cycling. Specifically, erosion can result in lateral fluxes of nitrogen and phosphorus that are similar in magnitude to those induced by fertilizer application and crop removal. Furthermore, the translocation and burial of soil reduces decomposition of soil organic carbon, and could lead to long-term carbon storage. The cycling of carbon, nitrogen and phosphorus are strongly interrelated. For example, erosion-induced burial of soils stabilizes soil nutrient and carbon pools, thereby increasing primary productivity and carbon uptake, and potentially reducing erosion. Our analysis shows soils as dynamic systems in time and space. Introduction The potential impact of soil processes on the biogeochemical cycling of carbon, and nutrients such as nitrogen and phosphorus, remains one of the great uncertainties in our knowledge of global climate change1, 2. Beginning with the pioneering work of Stallard3, scientists have become increasingly aware that lateral fluxes induced by soil erosion are of key importance in the global carbon cycle. So far, work on the influence of erosion on elemental cycles has focused on nutrient losses and effects on primary productivity; the influence on global nutrient and carbon cycles has yet to be studied in detail. In this Progress Article we estimate the impact of soil erosion on the carbon, nitrogen and phosphorus cycles using global datasets, and identify interactions between erosion and elemental cycles. Nutrient mobilization Several attempts have been made to estimate soil and carbon erosion rates associated with agriculture in recent years. From a critical analysis of these estimates, we calculate that sediment flux due to water erosion is about 28 Pg yr−1, and that a further ~5 Pg yr−1 and ~2 Pg yr−1 of sediment are mobilized by tillage and wind erosion, respectively, leading to a total sediment flux of about 35±10 Pg yr−1 (see Supplementary Information S1). This corresponds to an agricultural carbon erosion flux of 0.5±0.15 Pg C yr−1. Furthermore, we estimate that 0.08±0.02 Pg C is delivered to river systems by water erosion each year. To estimate the flux of nitrogen associated with erosion, we combine spatial estimates of soil erosion with global soil nitrogen data4 (see Supplementary Information S1). We estimate that around 23–42 Tg of nitrogen is moved by erosion each year. Lateral fluxes of nitrogen due to erosion are on the same order of magnitude as the 112 Tg of nitrogen applied to agricultural land in the form of chemical fertilizers each year5, the 75 Tg of nitrogen removed in harvested crops each year6, and the estimated riverine fluxes of particulate nitrogen, which range between 23 and 30 Tg N yr−1 (refs 7,8). We estimate that soil erosion is responsible for a flux of 2.1–3.9 Tg of organic phosphorus per year, and 12.5–22.5 Tg of inorganic phosphorus per year (see Supplementary Information S1). However, owing to the limited availability of global soil phosphorus data, these estimates are uncertain. Global mean phosphorus fluxes are considerably lower than the 40 Pg of phosphorus stored in soils globally9, but are similar in magnitude to crop uptake6 (14 Tg yr−1) and fertilizer phosphorus additions to agricultural land (~18 Tg yr−1). However, in some parts of the world, erosion-induced fluxes of phosphorus exceed phosphorus additions (Fig. 1), placing soil fertility and food production under increased strain. Figure 1: Global fluxes of sediment, nitrogen and phosphorus. a, Shaded areas show the global distribution of sediment fluxes derived using methods described in Supplementary Information S2. Bars show the continental fluxes of nitrogen and phosphorus by water and tillage erosion compared with fertilizer use6. b, Global fluxes of nitrogen and phosphorus (Tg yr−1) due to fertilizer input, erosion and crop uptake. Consequences for the carbon cycle Soil erosion encompasses soil mobilization, transport and deposition. Understanding erosional effects on the carbon cycle requires consideration of all three phases. When soil material is mobilized, soil structure is at least partially disrupted. Laboratory experiments indicate that sediment mobilization could result in a significant increase in the rate of soil organic carbon (SOC) mineralization, during, or shortly after, mobilization; this could lead to the loss of over 20% of the total SOC as carbon dioxide10. When considering the potential effect of transport on SOC mineralization, a distinction should be made between SOC deposited in a local depositional store after being transported over a relatively small distance (<500 m) by water or tillage over a short time (<1 day), and the fate of SOC that is delivered to rivers. Field observations indicate that the additional SOC mineralization that occurs during transport of soil over land is relatively unimportant: erosion–deposition simulations based on 137Cs inventories show that the carbon inventory found at depositional sites is inconsistent with significant mineralization during the transport phase11. Also, recent observations under field conditions suggest that SOC losses from soil that is re-deposited after a short transport phase are relatively low (<2.5% of eroded SOC), and therefore not very significant for the global carbon budget12. On the other hand, a large amount of SOC that is delivered to rivers may be mineralized within the river system13. Understanding the impact of erosion on the carbon cycle also requires consideration of the longer-term effects. Recent work suggests that erosion can increase both the emission and sequestration of carbon. The disruption of soil structure during erosion may lead to the immediate release of carbon dioxide. Enhanced emissions over longer time frames are associated with a reduction in the capacity of eroded soils to support plant growth14, resulting in lower carbon inputs through plant and root matter15. Erosion could also result in carbon sequestration16. Erosion leads to the mixing of carbon-poor subsoil into the plough layer. If the newly exposed mineral surfaces bind organic matter, soil carbon inventories may increase. The promotion of carbon sequestration by erosion relies on reduced rates of SOC decomposition, owing to the burial of sediment in depositional environments. Although the mechanisms that contribute to the reduction in decomposition at depth17 have only recently received attention18, the burial of pedogenic carbon at sites of deposition has repeatedly been shown to stabilize soil carbon over timescales of several decades, leading to reduced emissions of carbon dioxide11. Furthermore, mineralization can be actively suppressed in depositional environments where net primary production is greater than that in the source areas. For example, the influx of low-carbon sediments into wetlands and lowland valley bottoms may stimulate net carbon sequestration by diluting the concentration of soil carbon3. Overall, the extent to which mobilization and deposition lead to carbon storage is critically dependent on how much of the depositional accumulation is replaced by newly produced plant-derived soil carbon at eroding sites16 — dynamic replacement of soil (Fig. 2). Figure 2: Interplay between soil erosion, land use/soil management and carbon cycling at sites of erosion. The blue shaded area reflects possible combinations of carbon residence time (1/decomposition rate) and erosion rates as a function of land use/management. The numbers (g C m−2 yr−1) and size of the circles represents the maximum size of the carbon sink (positive, green) or source (negative, red) (see Supplementary Information S2). For croplands, the data represent high-input systems (HI, low sensitivity of yield decline to erosion, 4% per 0.1 m erosion) and low-input systems (LI, high sensitivity of yield decline to erosion, 15% per 0.1 m erosion). Full size image (25 KB) The removal of soil by erosion also brings the subsoil and parent material closer to the surface. There is increasing empirical and theoretical evidence that erosion is associated with increased rates of chemical weathering of silicate-rich parent material under steady-state conditions19, 20. As the weathering of silicate minerals consumes carbon dioxide, it seems likely that there may also be a link between erosion-induced weathering and the consumption of carbon dioxide, although the flux is likely to be small. In contrast, where parent materials are calcareous, accelerated weathering may result in carbon dioxide release to the atmosphere. For example, in the Canadian prairies it has been estimated that 10% of the carbonates acidified during erosion may be released as carbon dioxide, resulting in an estimated carbon loss of 0.12 to 1.2 Mg ha−1 yr−1 (D. A. Lobb, D. L. Burton, M. J. Lindstrom & D. C. Reicosky, unpublished observation). Impact on nutrient cycles Research into the impact of erosion on nitrogen and phosphorus cycling has hitherto focused on the assessment of nutrient mobilization and the delivery of nutrients to aquatic ecosystems. Less is known about the influence of erosion on nitrogen and phosphorus cycling within terrestrial environments. Soil organic matter contains large quantities of nitrogen and phosporus. Thus enhanced mineralization of soil carbon owing to soil mobilization21 will lead to a relative increase in dissolved nitrogen and phosphorus. These dissolved forms will be more accessible to biota than particulate or organic forms. On the other hand, burial and preservation of deposited carbon will lead to the stabilization of organic nitrogen. The stability of nitrogen in depositional environments may be high, and primarily determined by the rate of carbon mineralization. This could explain why C:N ratios in topsoils are remarkably constant within a given ecological context, and why palaeosol investigations report similar C:N values to those seen in present-day soils22. At eroding sites, dynamic replacement of carbon will also lead to the stabilization of nitrogen. However, nitrogen may also control carbon cycling: in some environments, biomass production and hence dynamic replacement may be directly limited by nitrogen availability23. Data from the Chinese loess indicate that buried soil phosphorus also remains relatively stable over long (>10 kyr) periods of time24. However, C:P ratios in soil organic matter show a larger variation than C:N ratios. Furthermore, a significant fraction of the phosphorus reservoir in soils is stored in inorganic form. Thus, the erosional effects on phosphorus cycling will be less tightly coupled to carbon cycling. Given that phosphorus is strongly bound to the mineral and organic soil fractions, we may tentatively assume that the evolution of phosphorus inventories in soils will be directly proportional to the amount of soil that is either mobilized or deposited. Indeed, erosion is an important mechanism for the decline in soil phosphorus levels over longer time periods25. This is not only due to the physical removal of phosphorus, and to the exposure of subsoil with lower phosphorus contents, but also to the interaction between erosion rates and chemical weathering (see above). As phosphorus levels decline over time, the phosphorus in the soil profile changes from a mix of mineral, occluded, non-occluded and organic forms to a mix dominated by organic and occluded forms25. In depositional sites, sediment can be an important source of phosphorus. In Hawaii, for instance, phosphorus limitation of forest growth on old soils is partially alleviated by dust deposition26. At sites where erosion dominates and inputs of nitrogen and phosphorus are low, primary production declines exponentially as erosion increases27, thereby reducing the potential for dynamic SOC replacement by vegetative material. The reduction in primary productivity is not only due to the removal of nutrients, but also to the degradation of soil structure and, critically, to a reduction in the availability of water as soil thickness declines. More subtle interactions may also take place. Evidence suggests that water erosion causes sediment carbon, nitrogen and phosphorus concentrations to be enriched, relative to the parent soil, to different extents during sediment mobilization and deposition28. Consequently, it is likely that the relative abundance of carbon, nitrogen and phosphorus in soils will change depending on the relative selectivity of mobilization and deposition processes: enrichment associated with water and wind erosion is likely to be greater than that for tillage erosion. Furthermore, the loss of carbon, nitrogen and phosphorus from mobilization sites might set in motion a degenerative feedback, whereby associated declines in plant productivity further increase erosion vulnerability and hence nutrient loss. Indeed it is well established that the resistance of soils to erosion is closely linked to the stabilizing influence of organic matter29 and vegetation cover30. Conversely at deposition sites, nutrient and carbon contents may rise, leading to greater primary productivity and a positive feedback on soil fertility, plant growth and resistance to erosion. Implications for soil function Erosion-induced changes in the cycling of carbon, nitrogen and phosphorus may influence a range of soil processes31, 32. For example, changes in the relative availability of carbon and nitrogen in soil organic matter is a primary regulator of microbial nitrogen mineralization–immobilization dynamics, and hence plant nitrogen supply33, 34. Changes in the N:P ratio of soil organic matter are also known to have significant consequences for decomposition, nutrient cycling and plant production35, 36. These feedbacks are likely to be of greatest significance in nutrient poor environments, such as the nutrient poor soils of Africa and Australia. In these regions, soil erosion associated with reduced vegetation cover and the loss of soil carbon can trigger catastrophic shifts to a severely degraded state15. The acceleration of erosion by these mechanisms may precipitate land-use change37, which itself changes the rate of biogeochemical cycling, thereby influencing atmospheric composition and climate change38, and further disrupting carbon, nitrogen and phosphorus cycling. Our analysis shows that agricultural landscapes are far from static: the accelerated rates of erosion experienced at present are causing major modifications to the terrestrial carbon, nitrogen and phosphorus cycles. We need to consider soils as mobile systems to make accurate predictions about the consequences of global change for terrestrial biogeochemical cycles and climate feedbacks. This is of primary importance, given that the imbalance between carbon and nutrient fluxes owing to erosion threatens the sustainability of food production and human welfare in many parts of the world.