Effects Of Agroforestry Or Controlled Burning On Soil Enzyme Activity In Eucalyptus Grandis Plantation

Agroforestry is a typical model in cooperative developing of forest and agriculture in the hill regions, but many problems such as land competition and low balances between forests and crops were always presented. Controlled burning is the traditional soil preparation before plantation, but it is questioned for the loss of soil, water and nutrients. These questionable issues could be mainly attributed to the unclear effects of agroforestry model and controlled burning on soil quality. Soil enzyme activity is the sensitive indicator in soil quality due to the relationships with material cycling and soil biological characters. However, little information has available on effects of agroforestry model and controlled burning on soil enzyme activity. Therefore, the Eucalyptus grandis plantation, which rapid developed and had the biggest area in Sichuan hilly regions, was selected. Soil enzyme activity and related nutrients was investigated in the growing season (July,2009), the end of growing season (October,2009) and spring (March,2010) under a representative agroforestry model and a plantation with controlled burning. The results indicated that,1) Lower soil bulk density and P content but higher content of available P, NH4-N and NO3-N were observed in the plantation of agroforestry model compared with pure E. grandis plantation. The content of available P and NO3-N displayed the lowest value in spring and the highest value in growing season, but the highest content of NH4-N was in spring and lowest in the end of growing season.2) Compared with pure E. grandis plantation, agroforestry model showed lower soil intervase activity, but higher activity of urease and phosphatase. The investigated hydrolases activities did the similar seasonal patterns in both pure plantation and agroforestry model. Invertase activity showed highest value in July and lowest in October. The highest activity of soil acid phosphatase was detected in July and lowest value in March, but lowest neutral phosphatase activity was observed in October.3) Both soil peroxidase and catalase activities were higher in agroforestry model than that in pure E. grandis plantation. Both plantations displayed the highest value in July and lowest value in March. However, although the highest catalase activity was observed in March in both plantations, the lowest value was in October in pure E. grandis plantation, the lowest value was in July in agroforestry model.4) Controlled burning significantly decreased soil pH, the decreased percentages were 10.2% and 6.8% in the soil layer with 0-20 cm and 20-40 cm, respectively. Controlled burning also decreased the content of soil organic carbon and N, but had insignificant effects on P content.5) Although the plantation at 3 months after controlled burning showed lower soil available P content, but soil available P content was higher at 6 months and 1 year after controlled burning compared with the plantation with no-burning. Soil P content showed a continually increase tendency in controlled burning plantation, but which showed an obvious decrease and then increase tendency in no-burning plantation. Soil NH4-N content significantly increased at 3 months after controlled burning, but there were few changes during other period. In addition, controlled burning treatment increased soil NO3-N content.6) 3 months and 6 months after controlled burning, soil invertase, urease and neutral phosphatase activities were lower in the plantation with controlled burning compared with no-burning plantation. Soil invertase activity showed an increase and then decrease tendency in no-burning plantation. Soil urease activity displayed an increase tendency in 0～20 cm, although which showed an increase and decrease tendency in 20-40 cm. Controlled burning decreased the acid phosphatase activity, and showed a increase and decrease tendency.7) Soil peroxidase activity was lower in the plantation with controlled burning compared with no-burning plantation, which showed a gradual decrease tendency. Soil catalase activity was higher in 0～20 cm in the plantation with controlled burning compared with no-burning plantation except for at 3 months after controlled burning. However, soil catalase activity was higher in 20～40 cm in the plantation with controlled burning compared with no-burning plantation only at one year after controlled burning.In conclusion, soil invertase, urease, phosphatase, peroxidase and catalase activities were higher in agroforestry model compared with pure E. grandis plantation, although soil invertase activity was lower. This suggested that agroforestry model might have higher soil quality and more efficient material cycling. Furthermore, controlled burning significant decreased soil pH and nutrients content, and soil enzyme activities, but at one year after burning, soil enzyme activity was higher in the plantation with controlled burning compared with no-burning plantation. This implying that controlled burning could have a negative effect on soil nutrients and enzymes in the earlier period, but it could improve soil quality by increasing soil enzyme activity. The present results could provide scientific data in reasonable managing E. grandis plantation and constituting balanced E. grandis plantation.