Abstract:

Objective: To provide theoretical reference for afforestation and its stand management in key regions of Tibet, we investigated the difference and changing trends of soil physicochemical properties between planted region and its adjacent area where without planting in the mid-watershed of “One River and Two Tributaries”, Tibet. And the understanding of soil fertility for artificial afforestation was improved systematically to there. Method: In the July to November of 2016, soil samples from four layers (0?20, 20?40, 40?60, 60?80 cm) at the depth of 0?80 cm were surveyed for 54 couples of plots which contained artificial forest (planted region) and its control plot (adjacent to artificial forest, with the same initial conditions, CK). And the soil physicochemical properties such as soil organic matter(SOM), total nitrogen(TN), total phosphorus(TP), total potassium(TK), alkaline nitrogen(AN), available phosphorus(AP), available potassium(AK), soil density(BD), soil mass water content(MWC), pH, and soil particle-size distribution(PSD) were tested. The methods of Wilcoxon signed-ranks Test, soil nutrient classification, PCA were used to analyze and compare the effects of artificial afforest on soil physicochemical properties. Result: Comparing with CK, the BD, TP, and AP for artificial forests in the 0?80 cm soil layer decreased obviously by 6.0%, 10.5%, and 36.5%, respectively. And MWC, TN, SOM increased obviously by 22.4%, 28.6%, 16.8%, respectively. AP varied largest and TN had the highest increase. pH, AK, TK, AN, and PSD changed insignificantly. More than 63.1% of the plots were in slightly deficient level or more worse levels for nitrogen, phosphorus, potassium, and soil organic matter of the CKs, and TN was in the worst nutrient classification. That percentage became to more than 73% for the artificial forests, and AP was in the worst nutrient classification. The plots’ proportion of AP for transferring from no deficient level to deficient level was also the highest. For CKs and artificial forests, the value of soil nutrient classification were ranked as TP>AK>AP>TK>SOM>AN>TN(the nutrient status is better on the left of ‘>’ than that on the right, the same below), TP>AK>TK>TN>SOM>AN>AP, respectively. The soil fertility changed insignificantly, which was comprehensive influenced by SOM, TN, and sand content. Conclusion: In the mid-watershed of “One River and Two Tributaries”, where is the key region for artificial afforestation in Tibet, the artificial afforestation has a obviously influence on the soil physicochemical properties, such as BD, MWC, TN, TP, AP, and SOM. For the artificial forests, the soil physicochemical conditions with soil density, soil mass water content, soil organic matter, nitrogen become better. While the nutrient condition of phosphorus becomes worse. The soil is mostly lack of AP for artificial forests, and its severest nutrient limitation is phosphorus, which is nitrogen for CK. The improvement of soil fertility is limited by AP. Whether afforestation or not, the soil nutrients are generally deficient in there. The soil fertility management should be strengthened for artificial afforestation in Tibet, especially focusing on enhancement the total level of soil nutrients. To improve the afforestation effect and soil fertility, the use of organic fertilizer can be strengthened before afforestation, and the use of phosphate fertilizer can be highlight in the subsequent management.

Key words: artificial afforestation, soil nutrient, nutrient classification, soil fertility, factor scores