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Microbial C/N/P stoichiometry ratios are of interest because of important ecosystem fluxes of C, N and P, such as mineralization and immobilization, and they can help to characterize the cycling of soil elements, especially regarding soil C sequestration. However, it is not clear how soil microbial C/N/P stoichiometry influences C cycling. In this study, the concentrations of microbial biomass C, microbial biomass N, microbial biomass P, and soil C fractions were measured in 45-, 40- and 25-yr-old black locust (Robinia pseudoacacia L.) forest soils and a sloped cropland. The results showed that soil organic C (SOC) concentrations or stocks and the percentage increment of the SOC stocks were highest under 45-yr-old black locust (RP45a). The concentrations of each C fraction in the black locust forest soils (45a, 40a and 25a) were higher than that of sloped cropland in the 0- to 30-cm soil profile. The concentrations of microbial C, N and P in RP45a were also higher by an average of 29.6 to 232.9, 9.2 to 17.9, and 0.6 to 1.1 mg kg-1, respectively, compared with the same concentrations in sloped cropland at the 0- to 30-cm soil depth. Redundancy analysis indicated that soil C fractions, especially for particulate organic C (53-2000 and >2000 mm), were significantly correlated with the microbial C/P and N/P ratios and the "best" model selection indicated that the microbial N/P and C/P ratios significantly changed with soil C fractions with regard to stand age and soil depth in the black locust forests. Therefore, the present study demonstrated that C fractions respond to microbial C, N and P stoichiometry after farmland-to-forest conversion and hence have the potential to affect C storage in the loess hilly region. [ABSTRACT FROM AUTHOR]

Changes in land use might affect the combined C, N and P stoichiometry in soil. The Grain-to-Green Program (GTGP), which converts low-yield croplands or abandoned lands into forest, shrub, and/or grassland, was the largest land reforestation project in China. This study collected the reported C, N and P contents of soil in GTGP zones to achieve the factors driving the changes in the C:N, C:P, and N:P values. The results showed that the annual average precipitation exerted significant effects on the C:P value, and on the N:P value became significant 20 years after the change in land use. The annual average temperature was the main factor affecting the C:N value during the first 10 years, while the annual average precipitation strongly affected this value afterwards. In addition, 'Redfield-like' interactions between C, N, and P in the soil may exist. A linear regression revealed significant positive correlations between the C:N, C:P, and N:P values and the restoration age, temperature, and precipitation after a change in land use. Therefore large-scale changes in land use under the 'GTGP' program might significantly affect the C:N, C:P and N:P ratios in soil. [ABSTRACT FROM AUTHOR]

Stimulation of root growth from mulching may enhance soil C fractions under maize (Zea mays L.). We studied the 5-yr straw (SM) and plastic film (PM) mulching effect on soil C fractions and maize yield compared with no mulching (CK) in the Loess Plateau of China. Soil samples collected from 0- to 10- and 10- to 20-cm depths after maize harvest in the fall, 2011 to 2015, were analyzed for soil organic C (SOC), particulate organic C (POC), potential C mineralization (PCM), and microbial biomass C (MBC). At both depths, all C fractions were 7 to 35% greater with SM than PM and CK. At 0 to 20 cm, SOC increased at 0.87 Mg C ha–1 yr–1 with PM and POC increased at 0.18 and 0.54 Mg C ha–1 yr–1 with SM and PM, respectively, from 2011 to 2015. Maize grain yield and aboveground biomass were 5 to 33% greater with PM and SM than CK. The PCM and MBC at all depths were negatively correlated with maize grain yield, but SOC and POC at 10 to 20 cm were positively correlated with estimated maize root residue returned to the soil. Plastic film mulching increased maize yield, but reduced soil C fractions compared with straw mulching. Because of favorable effect on soil C fractions and maize yield, straw mulching can enhance soil C sequestration by increasing labile and intermediate C fractions and sustain maize yield compared with no mulching under dryland cropping systems. [ABSTRACT FROM AUTHOR]

In the Loess Hill region of China, a large amount of arable land has been replaced by forest plantation, resulting in increased storage of soil organic C (SOC). To elucidate the mechanisms of SOC storage among the afforested lands, we separated SOC into four specific size/density fractions: coarse free particulate organic C (cfPOC), fine free POC (ffPOC), intra-microaggregate POC (iPOC) and mineral-associated organic C (MOC). Soils were collected from arable land and from areas with four tree species--robinia (Robinia pseudoacacia L.), poplar (Populus tomentosa Carriere), caragana (Caragana korshinskii Kom.) and buckthorn (Hippophae rhamnoides L.)-in 15-yr-old stands. The total soil C stock to a depth of 100 cm was in the order robinia > poplar > caragana = buckthorn and C increased by 7.9 to 18.2 Mg C ha-1 compared with the arable land. Stocks of the cfPOC, iPOC and MOC in the robinia and poplar land were also higher than that of the caragana and buckthorn land. Compared with the arable land, the MOC and cfPOC increased by 4.8 to 11.9 and 1.9 to 5.1 Mg C ha-1, respectively, in all of the afforested lands, and the iPOC only increased by 0.8 Mg C ha-1 in the robinia and poplar areas. However, the ffPOC increased by 0.9 Mg C ha-1 in all four afforested lands. The differences of the SOC fractions among the four afforested lands occurred primarily because the planting of robinia and poplar increased the cfPOC and MOC in the 0- to 100-cm soil profile, whereas caragana and buckthorn only increased cfPOC and MOC at depths of 0 to 20 cm and 0 to 60 cm, respectively. The total SOC fraction was composed of MOC (75.8%) > cfPOC (15.8%) > iPOC (4.0%) = ffPOC (4.3%). We conclude that the afforestation of former arable land with trees (robinia and poplar) can provide a greater increase in the SOC in the physical structure of the soil than shrubland (caragana and buckthorn), especially for the MOC. [ABSTRACT FROM AUTHOR]

This study investigated a typical secondary Betula platyphylla forest in the Ziwuling Mountains, Loess Plateau, China. In the sample plot, the DBH (diameter at breast height) class structure of B. platyphylla was bimodal. Individuals with small and large DBH values were abundant. The DBH structures of Quercus wutaishanica andPinus tabulaeformis were close to that of the logistic model, thus suggesting the increasing population of these species. B. platyphylla and Populus davidiana showed random spatial distributions at almost all scales. However, Q. wutaishanica and P. tabulaeformis were significantly clumped at small scales. B. platyphylla had a negative spatial relation with Q. wutaishanica at small spatial scales. P. tabulaeformis and Q. wutaishanica showed negative spatial correlations at small scales, but they had positive correlations at large scales. These results suggest that P. tabulaeformis and Q. wutaishanica shared habitat preferences at these scales. In the future, the secondary B. platyphylla forest in the Ziwuling Mountains in the Loess Plateau will probably change into a multi-species mixed forest (Quercus-Pinus mixed forest). Assisted restoration strategies must be employed to improve the regeneration dynamics of the forest in the long term. [ABSTRACT FROM AUTHOR]