Spatial Heterogeneity Of Soil Nutrients And Its Effect On Plants Of Old-fields In Loess Region

Using methods of pot and field experiment and spatial heterogeneity research of field nutrients, the spatial heterogeneity of soil nitrogen and phosphorus in old-field communities with different abandonment ages in Loess Hilly Region was demonstrated at community scale. The responses to heterogeneous fertilization of main plants in that local old-field community were analyzed, the impacts of fertilizer intensification and pattern and planting arragement on the growth of plants (above-ground biomass and below-ground biomass) were studied and the plasticities in response of single and mixed plantings root to fertilizer intensity and pattern were further discussed. The results are mainly as follows:(1) In the case of nutrient levels, old-fields in the study area are attributed by relative poor soils. In middle abandonment stage of succession, 0-20 cm and 20-40 cm available nitrogen and phosphorus increased while total nitrogen and phosphorus were the lowest, nutrient content in surface soil (0-20 cm) was significantly higher than that in sub-layer (20-40 cm). The variabilities of nitrogen and phosphorus were mainly originated from structural variation. Soil available potassium and nitrate nitrogen in 0-20 cm of the 6-7 years old-field, and subsurface soil nitrate nitrogen in 20-40 cm of the 25 years old-field had intermediate spatial correlation. In other old-fields, available nutrients in surface and subsurface soil had strong spatial correlations. Total nitrogen and phosphorus in surface and subsurface soil of old-fields with different abandonment ages had strong spatial correlation.(2) 0-20 cm and 20-40 cm available potassium and nitrate nitrogen showed the strongest spatial variability in earlier and middle abandonment stage of succession respectively. Spatial variability of available phosphorus and ammonium nitrogen in surface layer was the strongest in middle stage abandonment stage of succession; spatial variability of available phosphorus and ammonium nitrogen in subsurface layer was the strongest in early and middle stage of succession respectively. The spatial variability of total nitrogen in surface layer was stronger in the later abandonment stage of succession than that in the early and middle stage, which is contrary to the spatial variability of total nitrogen in subsurface layer. There was no apparent regular between the spatial variability of total phosphorus caused by spatial autocorrelation in the two soil layers and the increase of abandonment ages. Regarding about the index of spatial heterogeneity, significant differences were found among the 5 old-field communities, which implies that the principal ecological processes of total and available nutrients in surface and subsurface soil were assorted.(3) The field experiment was conducted by ingrowth core method. The results indicated that root biomass (RB), root biomass density (RBD) and specific surface area (SSA) increased in the middle and high nitrogen treatment than that in the control core, on the contrary, specific root length (SRL) decreased. The responses to heterogeneous fertilization were different among different plants. Artemisia scoparia was found had the highest increase percentage in root biomass, root biomass density and root length, reached about 654.98%, 65.98%, 392.42%, respectively. The decreasing percentage of Artemisia scoparia in specific root length was the largest, with a value of 56.11%, and Artemisia ginaldii had the largest increase in root surface area, about 990.14%.Heterogeneous fertilization by NP compound fertilizer showed that, Bothriochloa ischaemum had weak plastic response to phosphorus heterogenity under the same nitrogen intensification.(4) The pot experiment showed that fertilization intensity and pattern had different effects on root morphology, above-ground and below-ground biomass and the effects on species were different. In the pattern of homogeneous fertilization, Artemisia sacrorum, Setaria viridis- Lespedeza dahurica, Setaria viridis- Bothriochloa ischaemum had greater above-ground biomass in high fertility treatment than that in low fertility treatment and the increment was 2.180 g, 5.177 g, 10.785 g, respectively; Setaria viridis-Lespedeza dahurica and Setaria viridis- Bothriochloa ischaemum had greater below-ground biomass in high fertility treatment than that in low fertility treatment, the increment was 1.326 g and 2.054 g respectively. Artemisia sacrorum- Artemisia ginaldii and Setaria viridis- Artemisia sacrorum had greater above and below-ground biomass in treatments with heterogeneous soil nutrients than those in homogeneous soil under the same fertilization intensity, the increment was 9.163 g and 11.257 g , 5.190 g and 4.557 g, respectively.In the pattern of heterogeneous fertilization, Artemisia scoparia- Artemisia ginaldii, Artemisia ginaldii- Lespedeza dahurica, Artemisia ginaldii- Bothriochloa ischaemum, Lespedeza dahurica- Bothriochloa ischaemum had greater root biomass and fine root length in fertilized patch than those in control patch under the high fertility treatment, the increment was 0.020 g and 131.310 cm, 0.020 g and 140.385 cm, 0.088 g and 817.415 cm, 0.029 g and 505.031 cm. In the low fertility treatment, the increase of root biomass of Artemisia scoparia- Artemisia ginaldii, Artemisia scoparia-Lespedeza dahurica, Setaria viridis-Artemisia sacrorum, Setaria viridis-Lespedeza dahurica, Artemisia ginaldii-Bothriochloa ischaemum in fertilized patch than that in control patch was 0.024 g, 0.005 g, 0.003 g, 0.017 g , 0.024 g, respectively. The increase of root length of Artemisia scoparia-Setaria viridis, Artemisia scoparia-Artemisia sacrorum, Artemisia scoparia-Artemisia ginaldii, Artemisia scoparia- Lespedeza dahurica, Setaria viridis-Artemisia sacrorum, Setaria viridis-Lespedeza dahurica, Artemisia sacrorum-Bothriochloa ischaemum, Lespedeza dahurica-Bothriochloa ischaemum in fertilized patch than that in control patch was 84.794 cm, 41.619 cm, 448.595 cm, 68.142 cm, 384.522 cm, 583.764 cm, 45.162 cm, 62.025 cm, respectively. Artemisia sacrorum - Lespedeza dahurica had significant difference between homogeneous and heterogeneous fertilization in root biomass, Artemisia scoparia-Bothriochloa ischaemum had significant difference in fine root length.