Mapping Chinese annual gross primary productivity with eddy covariance measurements and machine learning.

Annual gross primary productivity (AGPP) is the basis for grain production and terrestrial carbon sequestration. Mapping regional AGPP from site measurements provides methodological support for analysing AGPP spatiotemporal variations thereby ensures regional food security and mitigates climate change. Based on 641 site-year eddy covariance measuring AGPP from China, we built an AGPP mapping scheme based on its formation and selected the optimal mapping way, which was conducted through analysing the predicting performances of divergent mapping tools, variable combinations, and mapping approaches in predicting observed AGPP variations. The reasonability of the selected optimal scheme was confirmed by assessing the consistency between its generating AGPP and previous products in spatiotemporal variations and total amount. Random forest regression tree explained 85 % of observed AGPP variations, outperforming other machine learning algorithms and classical statistical methods. Variable combinations containing climate, soil, and biological factors showed superior performance to other variable combinations. Mapping AGPP through predicting AGPP per leaf area (PAGPP) explained 86 % of AGPP variations, which was superior to other approaches. The optimal scheme was thus using a random forest regression tree, combining climate, soil, and biological variables, and predicting PAGPP. The optimal scheme generating AGPP of Chinese terrestrial ecosystems decreased from southeast to northwest, which was highly consistent with previous products. The interannual trend and interannual variation of our generating AGPP showed a decreasing trend from east to west and from southeast to northwest, respectively, which was consistent with data-oriented products. The mean total amount of generated AGPP was 7.03 ± 0.45 PgC yr-1 falling into the range of previous works. Considering the consistency between the generated AGPP and previous products, our optimal mapping way was suitable for mapping AGPP from site measurements. Our results provided a methodological support for mapping regional AGPP and other fluxes.

[Effects of straw incorporated to different locations in soil profile on straw humification coefficient and maize yield.] / æ–½å ¥ä¸åŒåœŸå±‚çš„ç§¸ç§†è æ®–åŒ–ç‰¹å¾åŠå¯¹çŽ‰ç±³äº§é‡çš„å½±å“.

Tillage and straw incorporation are important agricultural practices that can break plough layer and improve black soil fertility. The effects of tillage and straw incorporation on straw humification coefficient, soil organic carbon (SOC), and maize yield were investigated in a field experiment. Subsoil combined with straw incorporation in 20-35 cm soil layer (ST+S) could break plough layer and decrease the bulk density by 5.7%, 3.3% and 5.7% compared with traditional til-lage (TT), subsoil (ST) and traditional tillage combined with straw incorporation (TT+S) in six experimental years, respectively, and the best effects were observed in ST and ST+S treatments in the first expe-rimental year. The rate of straw decomposition was higher in 0-20 cm (72.0%) than in 20-35 cm (59.2%), and the straw humification coefficient in 0-20 cm and 20-35 cm soil la-yers reached the peak in first experimental year with 15.9% and 12.7%, respectively. Compared with initial soil sample, SOC and light fraction organic carbon (LFOC) of TT, ST and ST+S treatments in 0-20 cm soil layer was decreased in experimental years, but was increased by 2.9% and 12.4% within TT+S, respectively. SOC and light fraction organic carbon (LFOC) of ST+S in 20-35 cm soil layer was increased by 9.2% and 9.9%, respectively. The effect of field treatments on maize yield showed in a decreasing trend of ST+S>TT+S>ST>TT, effects of tillage and straw incorporation on maize yield could continue 3 and 6 years, respectively, indicating that tillage and straw incorporation had time effect. Therefore, straw incorporated into 20-35 cm soil layer based on tillage was an effective, sustainable agricultural practice of improving black soil quality.

Change in soil organic carbon between 1981 and 2011 in croplands of Heilongjiang Province, northeast China.

BACKGROUND: Soil organic carbon (SOC) is fundamental for mitigating climate change as well as improving soil fertility. Databases of SOC obtained from soil surveys in 1981 and 2011 were used to assess SOC change (0-20 cm) in croplands of Heilongjiang Province in northeast China. Three counties (Lindian, Hailun and Baoqing) were selected as typical croplands representing major soil types and land use types in the region. RESULTS: The changes in SOC density (SOCD) between 1981 and 2001 were -6.6, -14.7 and 5.7 Mg C ha(-1) in Lindian, Hailun and Baoqing Counties respectively. The total SOC storage (SOCS) changes were estimated to be -11.3, -19.1 and 16.5% of those in 1981 in the respective counties. The results showed 22-550% increases in SOCS in rice (Oryza sativa L.) paddies in the three counties, but 28-33% decreases in dry cropland in Lindian and Hailun Counties. In addition, an increase of 11.4 Mg C ha(-1) in SOCD was observed in state-owned farms (P < 0.05), whereas no significant change was observed in family-owned farms. CONCLUSION: Soil C:N ratio and initial SOCD related to soil groups were important determinants of SOCD changes. Land use and residue returning greatly affected SOC changes in the study region. To increase the topsoil SOCD, the results suggest the conversion of dry croplands to rice paddies and returning of crop residue to soils.

[Profile distribution and storage of soil organic carbon in a black soil as affected by land use types].

Taking soils in a long-term experimental field over 29 years with different land uses types, including arable land, bare land, grassland and larch forest land as test materials, the distribution and storage of soil organic carbon (SOC) in the profile (0-200 cm) in typical black soil (Mollisol) region of China were investigated. The results showed that the most significant differences in SOC content occurred in the 0-10 cm surface soil layer among all soils with the order of grassland > arable land > larch forest land > bare land. SOC contents at 10-120 cm depth were lower in arable land as compared with the other land use types. Compared with arable land, grassland could improve SOC content obviously. SOC content down to a depth of 60 cm in grassland was significantly higher than that in arable land. The content of SOC at 0-10 cm in bare land was significantly lower than that in arable land. Although there were no significant differences in SOC content at 0-20 cm depth between larch forestland and arable land, the SOC contents at 20-140 cm depth were generally higher in larch forestland than that in arable land. In general, SOC content showed a significantly negative relationship with soil pH, bulk density, silt and clay content and an even stronger significantly positive relationship with soil total N content and sand content. The SOC storage in arable land at 0-200 cm depth was significantly lower than that in the other three land use types, which was 13.6%, 11.4% and 10.9% lower than in grassland, bare land and larch forest land, respectively. Therefore, the arable land of black soil has a great potential for sequestering C in soil and improving environmental quality.