ABSTRACT:

Compared to all other terrestrial components, soil comprises the largest organic C stocks such that knowing the controlling factors and underlying processes can help to utilize its potential. Owing to the substantial spatio-temporal interplay at soil-plant-microbial interface, we have insufficient information to answer how different soil types and tree biomass interactions influence the surface layer soil microbial dynamics. Recognizing the uneven distribution of different soil fractions and temporal dynamics in moisture, soils of dryland agroecosystems are expected to have unknown trends in microbial biomass C. To understand how spatially variable tree biomass and temporal soil moisture trends affect soil microbial biomass, we performed periodic sampling runs in a flood irrigated Pecan orchard, located in Tornillo, Texas. The concerned study site has some unique spatial contrast developed by fluvial deposits on the regionally coarse soil fractions. The intriguing mosaic of contrasting soil textures has resulted in pecan trees with substantially variable tree biomass and created a spatial pattern of large and small trees adjoining the contrasting soil textures. In the present investigation, we have taken advantage of these features and chosen two pecan trees of contrasting biomass and collected four samples each on the north and south side of trees at a distance less than 5m from the tree base. The campaign involved surface soil sampling (0-10 cm) at every four-days interval for a period of 36 days and covering two irrigation events. Analysis of soil samples involved gravimetric soil moisture content, microbial biomass C and N. We found that soils associated with lower tree biomass (addressed as Pecan-fine) recorded significantly higher microbial biomass over the larger trees (addressed as Pecan-coarse). Furthermore, the regression analysis of soil microbial biomass versus gravimetric soil moisture content exhibited weak dependency of soil microbial biomass on the soil moisture variations. Overall, these results conclude that Tree size have substantial influence on the soil microbial biomass whereas the moisture dynamics have weakly affected the microbial biomass C.

ABSTRACT:

Drylands store a highly significant proportion of recalcitrant soil carbon, the SIC, primarily controlled by parent material and low aridity index (from 0.05 to 0.65) that favors more precipitation and lesser salt dissolution. Even though the annual net ecosystem carbon exchange (NEE) of these ecosystems is small compared to the grasslands and forests, their NEE is substantial and dominates with rapid climatic and land use change response of global land carbon sink. For instance, the agricultural practices such as saline water irrigation, fertilizer addition, tillage and mowing induced soil salinization, carbonate dissolution, nutrient loss, and organic matter decomposition as a response to land use change, can evidently play substantial role in affecting the unusually large soil C sink. Such a considerable response of soil C to the land use change arises the question about the relative contribution of possible factors altering the recalcitrant soil carbon pool (SIC) and affecting the development of soil organic carbon pool (SOC), with major implications for predicting and understanding the possible direction and magnitude of global carbon budget. Addressing these, we have collected in total one hundred, a meter deep soil cores from a flood irrigated Pecan orchard and analyzed the three depth ranges (surface: 0 to 10 cm, middle: 50 to 60 cm and bottom: 90 to 100 cm) for soil pH, EC, particle size, root biomass, soil organic (SOC) and soil inorganic carbon (SIC). Sampling points with prefixes 1 to 40 (1-0-10 to 40-90-100) were allocated to intensive sampling area, which were not randomized, while the sampling points with prefixes 41 to 90 (41-0-10 to 90-90-100) were allocated to non-intensive random sampling area. Sampling points with prefixes 91 to 100 (91-0-10 to 100-90-100) were ten random sampling points chosen at less than 1 m distance from ten other sampling points (chosen from intensive and non-intensive area), so, that a fine scale spatial variability across the study site can be estimated. Our results indicated soil texture as a master component of overall spatial variability, having good associations with SOC, SIC, root biomass and soil EC. A clear depth wise variation was recorded for the total root biomass, exhibiting an overall parabolic trend with maxima in middle depth range. However, the fine root biomass was found constant across different depth ranges. Soil EC showed mixed depth wise variation, with clear separations between the qualitative soil textural classes (sand and clay), exhibiting higher salt deposition in clay compared to sand. SOC exhibited a decreasing trend with depth ranges while SIC showed an overall constant trend across different depth ranges. Surface layer exhibited statistically similar SOC and SIC in different textural classes (i.e. sand and clay) while these differences become clear with middle and bottom depth range. CN ratio exhibited constant trend with depth range and textural variations, with overall similar mean CN ratio across different depth ranges and texture classes. Overall, this research indicates qualitative soil texture and depth ranges as strong predictors for variations in belowground carbon stocks (in soils and plant roots). However, there will be more to learn from the quantitative soil particle size analysis, soil carbon and EC analysis of the remaining sampling points.