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Fast-growing poplar plantations are considered a great benefit to timber production, but water availability is a key factor limiting their growth and development, especially in arid and semi-arid ecosystems. Super-absorbent polymers facilitate more water retention in soil after rain or irrigation, and they are able to release water gradually during plant growth. This study aimed to examine the effects of reduced irrigation (60% and 30% of conventional border irrigation) co-applied with super-absorbent polymers (0, 40 kg/ha) on root exudates, enzyme activities, microbial functional diversity in rhizosphere soil, and volume increments in poplar (Populus euramericana cv. 'Neva'). The results showed that 60% border irrigation co-applied with super-absorbent polymers significantly increased the content of organic acids, amino acids and total sugars in the root exudates, and the activities of invertase, urease, dehydrogenase, and catalase in the rhizosphere soil in comparison to conventional border irrigation without super-absorbent polymers. Meanwhile, this treatment also enhanced the average well-color development, Shannon index, and McIntosh index, but decreased the Simpson index. Additionally, the average volume growth rate and relative water content of leaves reached their maximum using 60% irrigation with super-absorbent polymers, which was significantly higher than other treatments. However, using 30% irrigation with super-absorbent polymers, had a smaller effect on rhizosphere soil and volume growth than 60% irrigation with super-absorbent polymers. Therefore, using an appropriate water-saving irrigation measure (60% conventional border irrigation with super-absorbent polymers) can help to improve enzyme activities and microbial diversity in the rhizosphere soil while promoting the growth of poplar trees. [ABSTRACT FROM AUTHOR]

Super absorbent polymers can absorb water up to a few hundred or even a few thousand times their own mass. The special physical structure, unique chemical composition and characteristics of high water absorption capacity allow super absorbent polymers to be widely used in water-saving agriculture, horticulture and other industries to relieve drought and reserve water for farmland, seedling protection and crop yield improvement. A large number of studies have showed that super absorbent polymer has the properties of water saving and drought resistance. But the application amount of super absorbent polymer is an important parameter to evaluate. Applying with the improper amount can result in bad effects, either no effect, or soil permeability compromised if excess super absorbent polymer is used. This can cause plant roots to have difficulty in breathing or to get rotten. In the meantime, border irrigation is also an efficient measure which has the advantages of low cost, simple operation and easy popularization. Similarly, the irrigation quantity is a vital factor. Because excess irrigation not only causes the waste of water resource, but also poses a threat to the groundwater. While insufficient irrigation may have no effect. Therefore, it is speculated that super absorbent polymer co-applied with border irrigation should be an interesting thing, and investigating their appropriate proportion has important theoretic value and practical significance. However, there is little information on the effect of border irrigation measure co-applied with super absorbent polymer on the forest-tree plantation. With the purpose of promoting water-saving irrigation and poplar yield, a field experiment of 5 treatments, i.e. CK (conventional border irrigation), I60 (60% conventional border irrigation quantity), I30 (30% conventional border irrigation quantity), I60S (60% border irrigation quantity co-applied with super absorbent polymers) and I30S (30% border irrigation quantity co-applied with super absorbent polymers) was designed and performed. The present study was conducted to determine the effects of different treatments on bulk density, microbial biomass carbon (MBC), microbial biomass nitrogen (MBN) and microbial activity in the forest soil as well as the growth of poplar. Results showed that in comparison to CK treatment, I60S treatment significantly reduced soil bulk density and obviously increased soil total porosity and capillary porosity, indicative of 6.62%, 11.19%, 16.45% and 7.97% decreases in soil bulk density compared to the treatments of CK, I60, I30 and I30S, respectively. The I60S treatment also evidently increased MBC and MBN contents and significantly enhanced microbial respiration, showing 13.89%, 24.24%, 41.38% and 12.33% increases in microbial respiration rate over the treatments of CK, I60, I30 and I30S, respectively. At the same time, the metabolic quotient was reduced by the I60S treatment, demonstrating 5.96%, 8.84%, 11.72% and 4.78% decreases, respectively. Additionally, the volume growth rate of the I60S treatment also reached the maximum, which had significant differences with other treatments (P<0.05). However, in the I30S treatment, less effect on the forest soil characteristics and the volume growth were observed than that in the I60S treatment. The correlation analysis showed that the volume growth rate was extremely significantly correlated with capillary porosity and metabolic quotient, and had significant correlation with soil bulk density, MBC and MBN contents and microbial respiration. In conclusion, the proper water-saving irrigation measure (60% border irrigation quantity co-applied with super absorbent polymers) is beneficial to improving soil physical environment and enhancing microbial activity in forest soil, as well as the growth of poplar. [ABSTRACT FROM AUTHOR]