Black carbon (BC) is the product of incomplete burning processes and a significant component of the passive soil organic carbon (SOC) pool. The role of BC in the global carbon cycle is still unclear. This study aimed to quantify and characterize BC in major grassland ecosystems of the world. Twenty‐eight representative soil profiles (mainly Mollisols) were sampled in the Russian Steppe, the U.S. Great Plains, the Argentinian Pampa, the Manchurian Plains in China, and the Chernozem region in central Germany. Black carbon contents were estimated using benzene polycarboxylic acids (BPCA) as a molecular marker, and indications about the origin of the BC were derived from bulk and compound‐specific δ13C analyses and radiocarbon dating of bulk soil organic matter (SOM). Our findings suggest that between 5% and 30% of SOC stocks consist of BC. Maximum BC contributions to SOC frequently were found at deeper parts of the A horizon with 14C ages younger than 7000 years BP; that is, incorporation of C as charred particles accompanied ecosystem development since the mid‐Holocene. Most of this BC formed from local vegetation, as indicated by a 13C isotope signature similar to that of bulk SOM. At some sites, also nonlocal sources contributed to soil BC, e.g., fossil fuel BC inputs at the German sites. Black carbon stocks were highest in Chernozems and lowest in Kastanozems. The Russian Steppe and Chinese Manchurian sites stored about 3–4 times more BC (around 3 kg m−2) than did the other sites because of thicker A horizons that were rich in BC. On a global scale, we estimate that steppe ecosystems contain between 4 and 17 Pg BC.
Crop productivity under dryland conditions is largely limited by soil water availability. Soil organic matter (SOM) contents have been found to be a reliable index of crop productivity in semiarid regions because it positively affects soil water‐holding capacity. Our objectives were to explain differences in wheat (Triticum aestivum L.) yields in response to SOM levels and related properties and to quantify the contribution of a unit increment of SOM content to soil productivity during 1991, 1992, and 1994 on a total of 134 production fields in the semiarid Argentine Pampas. Wheat yields were related to both soil water retention and total organic C (TOC) contents in the top layers (0–20 cm) in years with low moisture availability (1992 [r = 0.51, P < 0.01] and 1994 [r = 0.59, P < 0.01]), and were related to both total N and available P contents in a year without water deficit stress (1991 [r = 0.58, P < 0.01]). Wheat yields over all years were linearly related to TOC (r = 0.68, P < 0.01) when these contents were <17.5 g kg−. Dependence of wheat yields on soil water retention and on TOC contents under water deficit was related to the positive effect of these soil components on plant‐available water. In the absence of water deficit (1991), nutrient availability was the limiting factor. Losses of 1 Mg SOM ha− were associated with a decrease in wheat yield of approximately 40 kg ha−. These results demonstrate the importance of using cultural practices that minimize losses of soil organic C in the semiarid Argentine Pampas.
JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. The effect of single-tree influence areas on the physicochemical properties of the soil surface mineral horizon (0-18 cm) was studied in three stands of Pinus radiata D. Don introduced into Sierra de la Ventana, Argentina, grasslands 50 yr ago. Soil samples were taken at distances of 0, 1, and 2 m in transects from the tree to the periphery of the crown. Adjacent grassland soils with mollic epipedons were used as controls. Soil alteration was found to be highest near the trunk, with clear evidence of acid hydrolysis of primary silicates; the epipedon close to the trunk was classified as umbric rather than mollic. Decreasing values of pH, Ca, and exchangeable Mg, and increasing values of exchangeable H and Al, and also of fulvic acid-complexed Al, were registered from the grassland toward the axes of the trees. The soil properties analyzed fall into a distinctive spatial pattern of radial symmetry around each individual tree, with systematic and predictable variations, thus confirming the validity of the concept of "single-tree influence circles" for the study area. Each stand of P. radiata generates a patch of soil alteration within the undisturbed habitat matrix; the internal structure of these patches shows a radial pattern of different polypedons spatially and genetically associated with the inner ring of bark litter and the outer ring of leaf and twig litter. The present work shows that the introduction of P. radiata triggered changes in the evolutionary trend of the soils of such magnitude as to be reflected at the highest taxonomic level in soil taxonomy.
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