| Forest gap research has been paid more attention to the underground ecological system at present. In order to understand the influence of forest gap on soil carbon decomposition dynamics and the effect mechanism, the Platycladus orientalis L. plantation in Xuzhou was chosen as the research object. In this project, we mainly investigated the microbial metabolic activity and enzyme activity in the rhizosphere and bulk soil, as well as their rhizosphere effects(rhizosphere/bulk soil, R/S) along the horizontal gradient of forest gaps in large(L, 12 m in radius), middle(M, 8 m in radius) and small(S, 4 m in radius) sizes. In each gap size, three horizontal gradient points, namely forest gap edge(D1), 4 meters outside the gap(D2), and 8 meters outside the gap(D3), were chosen. The results showed that:(1) Forest gap reduced microbial biomass carbon(MBC) and nitrogen(MBN) in rhizosphere and bulk soil, which varied with the size of and the location in forest gaps. The rhizosphere MBC and MBN contents in the control plots(CK) are among 1528~3200 mg/kg and 156~900 mg/kg, respectively. Compared to CK, S gap significantly(p<0.05) increased the rhizosphere MBC content while other gaps showed no clear trends in the spring; all gaps generally reduced the MBC in the other three seasons, except that L gap showed a significant(p<0.05) positive effect in summer. Compared to CK, all sizes of gaps generally reduced the rhizosphere MBN content. Among three gap sizes, the MBC content in M gap and MBN in L gap were lower in the growing season, but an opposite trend was discovered in winter for both MBC and MBN. From D1 to D3 point, the rhizosphere MBC and MBN of all gaps generally increased first then fell. In all seasons, the bulk soil MBC and MBN content of all gaps were lower than those of CK. Their changes also followed a similar pattern among three gap sizes and along horizontal gradients within gaps.The influence of the forest gap on the rhizosphere effect of MBC, MBN varied with gap size. Compared with CK, forest gap generally improved the R/S of MBC and MBN in spring, but reduced those in summer and fall. In winter, the R/S of MBC increased while the R/S of MBN decreased in the gap, as compared to those in CK. The R/S of M gap was lower around the whole year.(2) Forest gap reduced microbial metabolic activity(AWC D) and increased microbial functional diversity, which varied with gap sizes and the positions in gaps. Compared to CK, with an AWCD value of 0.64 for rhizosphere microorganism, S and M gaps increased while L gap reduced the AWCD values. Forest gap improved the rhizosphere functional diversity index(H') and evenness index(E), up to 3.5 and 0.12 separately, while reduced the dominance index(Ds), down to 0.94. However, the direction of its effects on richness index(S) is unclear. From D1 to D3, the AWCD of three gaps declined first then increased, and the changes of the four kinds of diversity index were complex. Their values in D1 point were higher in M gap. Compared with CK, bulk AWCD of which is 0.73, forest gap decreased bulk AWC D. The value in S gap were higher than that in M and L gaps. In general, Forest gap increased the microbial functional diversity. From D1 to D3, the effect of forest gap on the AWCD and four kinds of diversity index varied with gap sizes and seasons.The effect of forest gap on the rhizosphere effects of AWCD and four k inds of diversity index varied with the sizes of gaps and positions within gaps. Compared with CK, forest gap increased the R/S(>1) of AWCD. Among three gap sizes, L gap had the greatest value. S gap primed the rhizosphere effect of S and E index, inhibited the rhizosphere effect of Ds index, while L gap showed an opposite trend. For M gap, it inhibited the rhizosphere effect of S index.(3) Four kinds of glycosidic enzyme activity was highest in the autumn, while invertase and amylase activity was higher in the spring and summer. The ?-glycosidase activity was the largest(can be up to 830 ug/g), while ?-galactose glucoside enzyme activity was the lowest(can be down to 17 ug/g) among four kinds of glycosidic enzyme. Compared with CK,forest gap reduced four kinds of glucoside enzyme activity in the rhizosphere in the growing seasons and increased amylase activity, but amylase activity reduced in spring, among 2~9 mg/g, and the trend of sucrose activity is opposite to amylase, up to 121mg/g. In four seasons, the rhizosphere enzyme activity was highest in the S gap and lowest in the M gap. Compared with CK, forest gaps generally reduced bulk soil enzyme activity in spring, increased the activity in summer and fall, while had no effects on the activity in winter. Among three kinds of gaps, the enzyme activity in bulk soil of M gap was lower. The change patterns of six kinds of enzyme activity from D1 to D3 point varied with the forest gap sizes and seasons.Compared with the R/S of invertase activity(>1) of CK, the R/S of forest gaps was increased in spring. The rank of the value became CK > S gap > L gap > M gap in summer and fall, while L gap = M gap > CK> S gap in winter. Compared with the R/S of amylase activity(>1, in growth season; <1, in winter) of CK, the rank of the value was CK > S gap > L gap = M gap in the growing seasons. Forest gaps in general decreased the R/S glucoside enzyme activity. The rhizosphere effect of enzyme activity responding to the position within forest gaps varied with the forest gap sizes and seasons.The study showed that the influence of forest gap on soil microorganism and enzyme activity highly depended on the size of gaps and the location gradient within gaps. The priming effects of forest gap on the rhizosphere effect of microorganism and enzyme activity showed dual feedback effects in four seasons and three gap sizes. Microorganism quantity and enzyme activity in M gap were generally low, while metabolic diversity was high. Therefore, the results suggested that M gap is the most conducive interference to enhance soil carbon immobilization, and to improve the ability of Platycladus orientalis L. plantation to cope with global warming. |
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