Linkages between soil carbon, soil fertility and nitrogen fixation inAcacia senegalplantations of varying age in Sudan

Abstract

BackgroundOver the last decades sub-Saharan Africa has experienced severe land degradation and food security challenges linked to loss of soil fertility and soil organic matter (SOM), recurrent drought and increasing population. Although primary production in drylands is strictly limited by water availability, nutrient deficiencies, particularly of nitrogen (N) and phosphorus (P), are also considered limiting factors for plant growth. It is known that SOM (often measured as soil organic carbon (SOC)) is a key indicator of soil fertility, therefore, management practices that increase SOM contents, such as increasing tree cover, can be expected to improve soil fertility. The objectives of this study were to investigate the effect of Acacia senegal (Senegalia senegal) trees on soil nitrogen, phosphorus and potassium (K) in relation to SOC, the potential of A. senegal for N2 fixation, and to identify possible N and P ecosystem limitations.MethodsSoil nutrient (total N, P, K and available P and exchangeable K) concentrations and stocks were determined for the 0–10, 10–20,20–30 and 30–50 cm layers of A. senegal plantations of varying age (ranging from 7 to 24-years-old) and adjacent grasslands (reference) at two sites in semi-arid areas of Sudan. At both sites, three plots were established in each grassland and plantation. The potential of A. senegal for N2 fixation in relation to plantations age was assessed using δ15N isotopic abundances and nutrient limitations assessed using C:N:P stoichiometry.ResultsSoil concentrations of all studied nutrients were relatively low but were significantly and directly correlated to SOC concentrations. SOC and nutrient concentrations were the highest in the topsoil (0–10 cm) and increased with plantations age. Acacia foliage δ15N values were >6‰ and varied little with plantations age. Soil C:N and C:P ratios did not differ between grassland and plantations and only 0–10 cm layer N:P ratios showed significant differences between grassland and plantations.DiscussionThe results indicated that soil fertility in the Sahel region is strongly related to SOM contents and therefore highlighting the importance of trees in the landscape. The higher mineral nutrient concentrations in the topsoil of the plantations may be an indication of ‘nutrient uplift’ by the deeper roots. The high foliar δ15N values indicated that N2 fixation was not an important contributor to soil N contents in the plantations. The accretion of soil N cannot be explained by deposition but may be related to inputs of excreted N brought into the area annually by grazing and browsing animals. The soil C:N:P stoichiometry indicated that the plantations may be limited by P and the grasslands limited by N.

Stable carbon and nitrogen isotopic composition of leaves, litter, and soils of various ecosystems along an elevational and land-use gradient at Mount Kilimanjaro, Tanzania

“…This should be in contrast with the typically altered nutrient dynamics of disturbed systems, particularly those under agricultural management (Wang et al, 2018). Low fractionation factors in δ 13 C are commonly reported between plant material and topsoils in natural systems, mainly because of the relatively limited humification of recent organic matter prevalent in topsoils (Acton et al, 2013;Wang et al, 2018). Thus, we hypothesized that if C inputs and outputs were roughly in balance, then the difference in δ 13 C values between plant material and topsoil would be smaller in undisturbed sites compared to managed or disturbed sites.…”

Enhancement of Soil Organic Carbon by Acacia Mangium Afforestation in Southeastern Region, Vietnam

“…Our findings revealed that soil TN and AN were strongly correlated with SOC (Table 2). In many ecosystems, it is well known that soil C and N are two closely associated biogeochemical processes (Hagedorn et al, 2003;Abaker et al, 2018). As one of the most essential nutrients for plant healthy growth, the quality and quantity of available N could impact the biomass productivity and the amount of plant litter, which is the dominant source of organic C (Bronson et al 2004;Sam et al 2006).…”

Changes in soil carbon, nitrogen, and phosphorus contents, storages, and stoichiometry during land degradation in jasmine croplands in subtropical China

“…Barren land together with monsoon climate with heavy rains results in a not-adequate combination to the accumulation of organic matter. The soil N input is mainly dependent on the return of plant residues, biological N fixation, and the input derived from the dry and wet deposition of the atmosphere (Abaker et al, 2018). However, due to the influence of artificial picking and other factors, the amount of jasmine plant residues is small, and the hilly topography is not conducive to the accumulation of N, so that the observed C and N storages were lower.…”