Response of Rice Harvest Index to Different Water and Nitrogen Management Modes in the Black Soil Region of Northeast China

Understanding the methods leading to rice yield increase is vital for sustainable agricultural development. Improving the harvest index (HI) is an important way to increase rice yield. To explore the effects of different water and nitrogen management modes on the rice HI in the black soil region of Northeast China, a field experiment was conducted in 2019 (Y1) and 2020 (Y2). Two irrigation methods, conventional flooding irrigation (FI) and controlled irrigation (CI), were established in the experiment, and four nitrogen application levels (0 kg/ha, 85 kg/ha, 110 kg/ha, and 135 kg/ha) were set during the entire growth period, named N0, N1, N2, and N3. The dry matter weight and the rice yield at the maturity stage were determined, and the HI was then calculated. The results showed that different irrigation modes and nitrogen application levels had significant effects on the rice HI. Under different irrigation modes with the same nitrogen application level during the two years, the comparison regular of HI was consistent. In Y1 and Y2, the HI of FN0 was 3.36% and 5.02% higher than that of CN0 (p < 0.05), and the HI of CN1 was 0.31% and 2.43% higher than that of FN1 (p > 0.05). The HI under CI was significantly higher than that under FI under N2 and N3 (p < 0.05), the HI of CN2 was 4.21% and 4.97% higher than that of FN2, and the HI of FN3 was 13.12% and 20.34% higher than that of CN3. In addition, during the two-year experiment, the HI first increased and then decreased with an increase in the nitrogen application rate under FI and CI. Under the FI treatments, the HI of N1 was the highest, and that of N2 was the highest under the CI treatments. A variance analysis showed that the irrigation pattern and nitrogen application level had significant interactions on the rice HI (p < 0.01), and the appropriate water and N management mode could increase rice the HI by 26.89%. The experimental results showed that the HI of the 110 kg/ha nitrogen application rate under CI was the highest, reaching 0.574 and 0.572, respectively, in two years. This study provides a data reference and theoretical support for realizing water savings, nitrogen reduction, and sustainable agricultural development in the black soil region of Northeast China.

Comparative Analysis of Arbuscular Mycorrhizal Fungal Communities between Farmland and Woodland in the Black Soil Region of Northeast China

The black soil region of northeast China is a critical production base for commercial grain in China. Arbuscular mycorrhizal fungi (AMF) are widely present in terrestrial ecosystems and play a vital role in ecosystem stability. Here, we investigated the diversity and composition of AMF communities in farmland and woodland from 20 sites in the black soil region of northeast China using Illumina MiSeq sequencing. The sequences were classified into 1 phylum, 1 class, 4 orders, 8 families, and 11 genera. Glomerales and Paraglomerales were observed as the most abundant order in farmland and woodland, respectively, and also belonged to abundant orders of the black soil region in northeast China, accounting for more than 90% of the total. Furthermore, Paraglomus, Claroideoglomus, and Glomus were the most abundant genera. Canonical correspondence analysis demonstrated the effect of soil pH, invertase, nitrogen, phosphorus, and soil organic carbon (SOC) contents on AMF community composition. Results from the correlation analysis revealed a reduction in AMF diversity with increases in SOC and phosphorus contents. These findings suggest AMF community composition varied with land use type (farmland and woodland), and provide a basis for protecting and utilizing AMF resources in the black soil region of northeast China.

Geostatistical Characterization of Soil Clay Patterns in a Typical Watershed of Cultivated Black Soil

Soil clay influences a range of processes, including vertical and lateral redistribution of water, runoff, and erosion. Knowledge of the spatial pattern of soil clay content (SCC) in cultivated soil region is important. The objective of this study was to determine the degree of spatial variability of SCC across depths in a cultivated black soil region in northeastern China. Data collected from each of 58 sites on a regular grid of 50 by 50 m were analyzed both statistically and geostatistically to describe the spatial distribution. The SCCs between different segments in soil profiles were significantly different. The 0- to 65-cm soil profile could be clustered into three segments: 0- to 30-cm, 30- to 35-cm, and 35- to 65-cm layers.The standard deviation (SD) and coefficients of variation (CV) for the SCC of the 30- to 35-cm segment were the largest. Experimental semivariograms of SCC were best fitted by spherical models. Nugget-to-sill ratios indicated a strong spatial dependence for SCC at other depths, except the 40- to 45-cm and 60- to 65-cm segments. The 35- to 40-cm layer had the largest spatial dependency compared with the other layers. Cross-validation of the kriging map indicated that prediction of SCC using semivariogram parameters was better than assuming the mean of the observed value for any unsampled location. The mapping displayed heterogeneity of SCC across the experimental site and revealed higher SCC close to the tail of an eroded gully and lower SCC neighbouring eroded gully margins. The fragmentation degree and clay-enriched patch amount increased from the near-surface down to 65-cm of depth, suggesting the higher evenness of SCC in the cultivated layer than that in the tillage pan.

Effect of Land Use Change on Gully Erosion Density in the Black Soil Region of Northeast China From 1965 to 2015: A Case Study of the Kedong County

Kedong County is typical of the black soil region of northeast China in being highly susceptible to accelerated soil erosion by gullying. Using data sourced from Corona satellite imagery for 1965, SPOT5 for 2005 and GF-1 for 2015, the spatial distribution of gullies in the research area was mapped. Land use data for 1965, 2005, and 2015 were obtained from the topographic map of 1954, and from Landsat images for 2005 and 2015. Over the last 50 years, the extent of gully erosion in the study area has increased markedly, most notably on cultivated land, while gully density rose from 2,756.16 m2/km2 to 14,294.19 m2/km2. Cultivating land on slopes, especially on slopes greater than ∼4°, may rapidly aggravate gully erosion. The greatest increases in gully density occurred in situations when cultivated land and other/degraded land were transformed, which gully erosion density increased by 49,526.69 m2/km2. Other/degraded land is the most vulnerable land in the study area, with the highest gully erosion density. In these cases, gully density initially increases and, although the “Grain for Green” project has been implemented, gully erosion density has not always declined in the recent past.

Distributions of Particle Sizes in Black Soil and Their Environmental Significance in Northeast China

In recent years, black soil has decreased and degenerated heavily due to complicated functions of natural and artificial factors. Hence, characterizing distributions of particle sizes in black soil and their environmental influencing factors is important for understanding black soil degradation. A total of 116 surface soil samples in the top 20 cm from a typical black soil region in northeastern China were collected, and the spatial distribution of particle size parameters were characterized. Particle size-sensitive components were extracted quantitatively using the log-normal distribution function, and their environmental implications were investigated. The contents of black soil mechanical composition ranged from 7.8% to 79.3% for clay, 17.7% to 80.3% for silt, and 0% to 73.7% for sand, respectively. Median particle size ranged from 1.71 to 142.67 μm, with a coefficient of variation of 60%, indicating silt accounted for the majority of the composition. Four environmentally sensitive components were identified, including long-distance transported airborne deposits of clay dust (C1), successions from local parent materials (C2), short-distance deposits of silt particles (C3), and a component strongly disturbed by human activities (C4). C1 and C2 had relatively low variations, with C1 exhibiting the smallest variation, and C2 contributing highest proportion, showing no significant differences across all samples. C3 widely existed across samples, suggesting common wind erosion within the black soil region. C3 and C4 varied spatially, which was caused by the low vegetation coverage and high human disturbance of agricultural topsoil. The results suggest that windbreaks should be encouraged to reduce wind erosion in the black soil regions.