Soil carbon sequestration in urban soils under different land uses in a semi-arid region: physico-chemical and biochemical factors influence
Abstract
Soils play a major part in carbon storage, thus providing a potential lever for climate regulation. There specific contribution is historically highly documented for natural soils and has also been more recently pointed out for more anthropized soils, i.e. Technosols, especially in urban environments. Still, carbon storage in soils is reversible and the temporal and spatial variability of SOC stocks in urban areas are determined by different biotic and abiotic factors. SOC content can be important and highly variable for urban soils and there is evidence of high levels of SOC in regularly irrigated and fertilized lawns. In order to improve urban management and ensure long-term storage of organic carbon in soil, this work proposes to determine the soil organic carbon stocks (SCOS) in urban soils according to different land use types. Physico-chemical and biochemical factors known to influence this stock are analyzed for a wide array of urban land uses. Twenty-six urban soils were sampled according to seven different land use types and analyzed focusing on the following physico-chemical and biological parameters: pH, total organic carbon (TOC) and total nitrogen (Ntot), available P (Pav), total fraction of the trace and major elements (TE, ME), alkaline-phosphatase (APHa), urease (URa), dehydrogenase (DHa) and β-Glucosidase (β-Glu) activities. The SOC, Ntot and Pav results showed no significant differences from the control, except for the peri-urban soils irrigated with wastewater (SAS-WWE) where SOC and Ntot were 3 times higher than the control, while it was 10 times higher for Pav. The same trend was recorded for the SOC stock, where the highest value was reported in SAS-WWE with 111.95 g C/dm². This may be due to the fertilizing power of wastewater rich in organic matter (OM) and mineral elements that has been used to irrigate these agricultural soils for several years. This particularity gives SAS-WWE a tendency to store more carbon in than any other type of use. This is further confirmed by significant and positive correlations (>0.5) observed for SOC, Ntot and Pav (r= 0.957; r= 0.664; r= 0.548, respectively). β-Glu activity was significantly higher for soil use types that showed a fairly high SOC stock values (r = 0.50). This is illustrated by a correlation between SOC and β-glucosidase, the enzymatic activity responsible for the decomposition of cellulose. Unlike APHa and URa, lower DHa value was observed in the differents land uses, where only the control have recorded the highest activity with 5.8 μg TPF g-1 dry soil h-1. A significant correlation between DHa and TOC is noticed (r=0.73). Therefore, high OM content indicates that the large part of the SOM consisted of easily decomposable organic material.