The impact of land use on CO2 intake of chemical weathering in a typical karst region: a case study of Xiaojiang watershed, Yunnan Province

Abstract：The weathering process of terrestrial strata consumes atmospheric/soil CO2, forming an important sink of atmospheric CO2. This study intends to highlight the relationship between CO2 intake by carbonate weathering and land use changes in a typical karst area at Xiaojiang watershed of Yunnan Province. The authors analyzed more than 600 groups of groundwater composition data collected from 25 monitoring points in 1982 and 2003. These monitoring points were chosen from 12 typical areas of land use conversion in the watershed that consisted of four change areas from forestland to cultivated land, three from unused land to cultivated land, four from unused land to forestlands and one from forestland to construction land at Xiaojiang watershed. CO2 intake strength (CIS,%) by carbonate chemical weathering is presumedly defined as the percent of carbon from atmospheric/soil CO2 (representation with HCO-3) in the total carbon during chemical weathering (representation with HCO-3) , and its formula is expressed as CO2 intake strength (CIS,%)=100×1-(Ca2++Mg2+)total waters)/ HCO-3 total waters. The estimated results of CIS changes of four land use conversions from 1982 to 2003 show that CIS of forestland (48.84%) is higher than that of unused land (47.66%). From 1982 to 2003 the average CIS decreases in these three land use change areas (unused land to cultivated land, forestland to cultivated land and forestland to construction land) were -7.85%, -8.59% and -1.66%, respectively. CIS increase of the change area from unused land to forestland was 1.74% on average. As for cultivated lands changed from unused land and forestland, construction land from forestland from 1982 to 2003, the ratios of Ca2++Mg2+/ HCO-3 in groundwater were from 0.510 to 0.589, from 0.515 to 0.601, and from 0.508 to 0.525, respectively. All the ratios increased. For the conversion of unused land into forestland, the ratio declined from 0.533 to 0.524. The more the ratio increased, the less CIS and vice versa. In addition, it is found that loss of HCO-3 is positively correlated to SO2-4+NO-3 concentrations (in mmol/l) measured in groundwater, SO2-4+NO-3 tends to increase while loss of HCO-3 increases. The more the loss of HCO-3 is, the less the CIS and vice versa. Adding N-fertilizer such as (NH4)2PO4, (CO(NH2)2), NH4NO3, (NH4)2SO4 after changing from unused land or forestland to cultivated land interferes or counteracts with carbonate weathering due to carbonic acid because such N-ferlitizers can become HNO-3 by nitrification. This results in a relative increase of Ca2+, Mg2+ and a relative decrease of HCO-3 in karst water. When this occurs, it lessens the CIS.