Abaker Wafa E, Berninger Frank, Saiz Gustavo, Pumpanen Jukka, Starr Mike
Department of Forest Sciences, Viikki Tropical Resources Institute, University of Helsinki, Helsinki, Finland.
Faculty of Forestry, University of Khartoum, Khartoum, Sudan.
PeerJ. 2018 Jul 10;6:e5232. doi: 10.7717/peerj.5232. eCollection 2018.
Over 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 () trees on soil nitrogen, phosphorus and potassium (K) in relation to SOC, the potential of for N fixation, and to identify possible N and P ecosystem limitations.
Soil 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 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 for N fixation in relation to plantations age was assessed using δN isotopic abundances and nutrient limitations assessed using C:N:P stoichiometry.
Soil 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 δN 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.
The 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 δN values indicated that N 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.
在过去几十年里，撒哈拉以南非洲经历了严重的土地退化和粮食安全挑战，这些挑战与土壤肥力和土壤有机质（SOM）流失、反复干旱以及人口增长有关。尽管旱地的初级生产受到水资源可用性的严格限制，但养分缺乏，特别是氮（N）和磷（P）的缺乏，也被认为是植物生长的限制因素。众所周知，SOM（通常以土壤有机碳（SOC）衡量）是土壤肥力的关键指标，因此，增加SOM含量的管理措施，如增加树木覆盖，可以预期会提高土壤肥力。本研究的目的是调查（）树木对与SOC相关的土壤氮、磷和钾（K）的影响，（）的固氮潜力，并确定可能的氮和磷生态系统限制。
在苏丹半干旱地区的两个地点，测定了不同年龄（7至24年）的（）种植园以及相邻草原（对照）0 - 10厘米、10 - 20厘米、20 - 30厘米和30 - 50厘米土层的土壤养分（全氮、磷、钾以及有效磷和交换性钾）浓度和储量。在两个地点的每个草原和种植园中都设立了三个样地。利用δN同位素丰度评估（）与种植园年龄相关的固氮潜力，并利用C:N:P化学计量学评估养分限制。
所有研究养分的土壤浓度相对较低，但与SOC浓度显著且直接相关。SOC和养分浓度在表土（0 - 10厘米）中最高，并随种植园年龄增加而增加。相思树叶的δN值>6‰，且随种植园年龄变化不大。草原和种植园之间的土壤C:N和C:P比率没有差异，只有0 - 10厘米土层的N:P比率在草原和种植园之间存在显著差异。
结果表明，萨赫勒地区的土壤肥力与SOM含量密切相关，因此突出了树木在景观中的重要性。种植园表土中较高的矿质养分浓度可能表明深层根系的“养分提升”。较高的叶片δN值表明固氮对种植园土壤氮含量的贡献不大。土壤氮的增加不能用沉积来解释，可能与每年通过放牧和啃食动物带入该地区的排泄氮输入有关。土壤C:N:P化学计量学表明，种植园可能受到P的限制，而草原受到N的限制。
