Effect of continuous cropping on basic physicochemical properties of inter-root soils of millets
Continuous cropping alters the soil microenvironment and has a certain effect on soil nutrients22. It is now widely recognized that the malignant change in soil physical and chemical properties is one of the main factors contributing to the occurrence of crop continuous cropping disorders23,24. Some studies have shown that with the increase in the number of years of continuous cropping of millets, the AK content of soil was significantly higher than that of the control rotation, and the soil pH of rotated millet fields was significantly lower than that of continuous cropping25. The results of the this paper also indicate that, without fertilization, the content of AK 3 years (T2) and 4 years (T3) millet crop was significantly higher than that of crop rotation (CK), AK includes water-soluble and exchangeable potassium, and the accumulation of AK in the soil produces a change in the osmotic pressure of the soil environment, which affects the uptake of K+ as well as other nutrients by the crop, thus affecting the growth and development of the crop26; meanwhile millet continuous cropping increased soil pH, the pH increased from 8.3 to 8.6 after 4 a, which may be related to the uptake of acidic anionic elements by the millet root system and root secretions27; the content of N and P in millet continuous cropping soils showed a tendency to decrease, and N decreased more quickly, due to the presence of leguminous plants in the CK treatment of the rotation, which reduced soil N consumption to some extent, and the soil N content of the CK treatment was relatively higher than that of the continuous crop treatment.
Soil nutrients are the ability of soil to provide nutrition for plant growth and to coordinate nutritional and environmental conditions, and are a comprehensive expression of various basic properties of the soil, and tillage practices, fertilization systems, and planting patterns all have an impact on soil nutrients28. Abundant N, P, and K elements in the soil is a prerequisite for achieving high crop yields29. Different crops have different preferences for the absorption and utilization of soil nutrients, and their root systems have different impacts on the physical and chemical properties of the soil; therefore, crop rotation of different crops can well coordinate the changes in soil physical and chemical properties and nutrients, and improve the effective utilization of soil nutrients. However, continuous cropping will consume a huge amount of a certain nutrient element in the soil, while absorbing and utilizing less of certain elements, leading to the accumulation of the seed element and affecting the overall fertility level of the soil30.
Effect of continuous cropping on inter-root soil enzyme activities in millets
Soil enzymes mainly come from plant root secretions and soil microbial activities, and participate in various soil biochemical processes, which are sensitive indicators for soil environmental quality monitoring31. Soil urease is a kind of specialized amidase, which can promote the hydrolysis of peptide bonds, and urease activity reflects the nitrogen supply status of soil to a certain extent32. This study showed that soil urease activity decreased year by year with the increase of years of continuous cropping. Continuous cropping reduces its activity probably because soil urease is mainly derived from microorganisms33, and the microbial populations of soil under continuous cropping for many years have changed, the urease activity is reduced. The urease activity of the CK treatment of the control rotation was higher than that of the continuous cropping treatment in all fertility periods, which may be due to the fact that the previous crop of the CK treatment was kidney bean, and the nitrogen fixation of legumes increased the soil N content, according to the relevant research, the N content in the soil was positively correlated with the soil urease activity34.
Soil ALP activity characterizes the fertility status of soil, especially phosphorus, and is a key enzyme in determining soil phosphorus conversion30. In this study, the soil ALP activity showed a gradual decreasing trend with increasing years of continuous cropping in millet, in which T3 was significantly lower compared with T1, while the mean values of ALP activity in both T1 and T2 treatments were greater than those in CK treatment during the whole fertility period. Short-term continuous cropping of millets had little effect on soil ALP activity, while long-term continuous cropping significantly reduced ALP activity.
Sucrase is an enzyme that catalyzes the hydrolysis of sucrose into glucose and fructose, which can increase the soluble nutrients in the soil and improve the biological activity of the soil, reflecting the degree of soil maturation31. This study showed that continuous cropping of millets reduced soil sucrase activity, and the longer the years of continuous cropping the lower the activity, which is consistent with the findings of Bai Yanru35 on potato. The reduction of sucrase activity makes the soluble nutrients in the soil decrease, and the soil maturation degree decreases, which is not favorable to the growth and development of the crop, which may be one of the reasons for the decrease of millet yield in continuous cropping.
Soil catalase catalyzes the decomposition of hydrogen peroxide to form water and oxygen, relieving the toxic effects of hydrogen peroxide on organisms and soil due to plant or microbial metabolism30. This study showed that continuous cropping reduced catalase activity, slightly in T1 and more significantly in T2 and T3 compared to CK, which is in agreement with the results of Gao Yang27 et al. on continuous cropping of wheat. With the inhibition of hydrogen peroxide hydrolysis by long-term continuous cropping in millet, the toxic effect of hydrogen peroxide in the inter-root soil was aggravated, which may be one of the reasons for the occurrence of crop failure in millet, and the inhibitory effect of short-term continuous cropping on the activity of catalase was not significant; crop rotation can effectively enhance the oxidative capacity of the soil, and increase the ability of hydrogen peroxide decomposition31.
Continuous cropping Obstacle is the result of integrated soil-microbe-plant-climate interactions, and there is also a mutual utilization relationship between the reaction sites and products of various soil enzymes36. Therefore, only from the soil enzyme activity of a single index change is still very difficult to fully explain the possible occurrence of continuous cropping obstacles from the mechanism, and the study of a single factor is difficult to fully and completely explain the situation of field experiments, long-term continuous cropping of millet on soil microbial taxa, soil physical and chemical properties and soil enzyme activity of the impact of further in-depth research is still needed.
Fertilization of millets cultivation
This study showed that under millet continuous cropping in the arid northwest region, without fertilizer application, millet yield decreases year by year, indicating that fertilizer application is still required to maintain soil fertility and increase yield considering the depletion of nutrients in the soil. In contrast, millets yields under the rotation treatment were significantly higher than the continuous treatment, which was related to the legume crop brassica in the rotation, whose root nitrogen-fixing rhizobacteria fixed a certain amount of nitrogen from the atmosphere to replenish the soil, thus increasing millet yields37. Therefore, crop rotation with leguminous crops should be practiced as much as possible in millet production.