Soil carbon storage  affected by long-term land use regimes and fertilizationin  manural loess soil

The storages of soil organic and inorganic carbon under contrasting land use regimes and fertilization were analyzed on a basis of a 20-year long term experiment in a manural loess soil. There were eleven treatments, fallow (FL), setaside (SL), unamended control (CK), nitrogen (N), nitrogen and potassium (NK), phosphorus and potassium (PK), nitrogen and phosphorus (NP), nitrogen phosphorus and potassium (NPK), wheat/maize straw (S) with NPK (SNPK) and dairy manure (M) with NPK (M1NPK and M2NPK). The results indicated that the storages of soil organic carbon in the soils of the FL and CK, with an equivalent mass to that of top 20 cm soil of CK, remained unchanged relative to the soil prior to the commence of the experiment (Initial soil, IniS), while the storages were significantly improved under the treatments of NP and SL with an annual increase of 347 kg/ha and 518 kg/ha, respectively. The storages of soil inorganic carbon for CK, NP, FL and SL were dropped dramatically compared to initial soil, particularly for NP, on which it is even significantly lower than those of other three treatments. For treatments where nutrient management treatments were applied in conjunction with the cropping regime of winter wheat and summer maize rotation (exclusive of FL and SL treatments), the storages of soil organic carbon in plough layer soils on an equivalent mass basis of all but NK treatments were markedly enhanced compare with those of CK and initial soil in a order of statistic significance as follow: M2NPK M1NPK SNPK NPK NP PK N, of which, the maximum increase was observed on M2NPK with an annual mean value of 944 kg/ha, and a minimum value of 127 kg/ha on N. Whereas considerable decreases of the soil inorganic carbon storage in plough layer soils on an equivalent mass basis were found in all treatments except for PK and M2NPK in comparison to initial soil value, this is probably due to the acidification of the soil as the consequence of either precipitation or fertilization. The fact that the storages of inorganic carbon in PK and M2NPK, in ca. top 20 cm soils on equivalent mass, remained stable relative to their initial soil values might suggest that soil organic carbon may convert to inorganic form under some circumstances. The results also exhibited substantial decline in soil bulk densities in the treatments those receiving no organic manure, this result implied that the soil carbon storage on an equivalent mass basis rather than the equivalent soil depth could be a better indicator of soil carbon changes.