Understorey management effects on N dynamics and availability in agroforestry systems

Abstract

The effects of understorey management on the dynamics and availability of N in agroforestry systems were studied at the Lincoln University agroforestry experiment. The objective of this research was to quantify the effects of understorey management on temporal changes and spatial variability of inorganic N, N mineralisation, nitrification and microbial biomass C and N in agroforestry systems.

The experiment was a split-plot, completely randomised block design with three replications. The main plots are Pinus radiata trees planted with an understorey of Yatsyn perennial ryegrass (Lolium perenne) pasture or a control treatment with no understorey (bare ground). In each treatment two transects were established (one 0.9 m north of the tree rows and the other midway (3.5 m) between the tree rows). Along each transect, three sampling sub-plots (1 x 1 m) were maintained for study. The sub-plots in the ryegrass plots were caged to exclude urine and excreta from grazing sheep.

One pair of intact soil cores was collected from both soil depths (0-10 and 10-20 cm) in each sub-plot on each sample date. Nine samplings were conducted from July 1997 to June 1998. Microbial biomass C and N contents (Chloroform-fumigation extraction method), net N mineralisation and nitrification rates (buried-bag method), and soil mineral N (KCI extractable NH₄⁺-N and NO₃⁻-N) levels were measured. Soil temperature was monitored by data loggers and moisture was measured weekly by TDR.

Ammonium and nitrate concentrations in the top soil (0-10 cm) layer were higher in the bare ground than in the ryegrass plots. Net N mineralisation exhibited obvious seasonal trends; the highest rates were found in winter when soil moisture content was generally greater than 25% of soil volume and soil temperature remained above 5°C. Net nitrogen mineralisation rate in the top soil layer ranged from 0.21 to 0.26 µg N g⁻¹ day⁻¹. Nitrification rate followed the same trend and varied from 0.21 to 0.40 µg N g⁻¹ day⁻¹; the nitrification rates were generally higher in the bare ground than in the ryegrass plots. The amount of microbial biomass C and N contents were greater in the ryegrass than in the bare ground plots in the top soil layer. The increased amounts of microbial biomass C and N in the rye grass plots were likely due to the availability of carbon substrates, derived from the decomposition of pasture roots. This relationship between microbial biomass and substrate availability suggests that a greater amount of N was immobilised by the microbial biomass in the ryegrass than in the bare ground plots.

The top soil layer of both understorey treatments had higher microbial biomass than the sub-soil layer which acts as an important source of inorganic N production. Nitrogen mineralisation and nitrification rates were also higher in the top soil than in the sub-soil (10- 20 cm) layer where there was more organic matter and higher microbial biomass C and N. Decomposition of pasture and tree roots and pine needle litter was the major source of organic matter input in the studied site. Soil temperature and moisture also played a major role in determining soil microbial activity and soil N availability.

Significant positive correlations among net N mineralisation, nitrification, microbial biomass and N, microbial C:N ratio, and soil moisture were observed in both 0-10 and 10-20 cm depths of the ryegrass treatment. These correlations provide support for the conclusion that the microbial biomass is an important regulator of soil mineralisation and nitrification processes. Correlations of the selected variables in the bare ground treatment were greatly influenced by the soil depths. However, soil ammonium and nitrate concentrations were negatively correlated with net N mineralisation, nitrification and microbial biomass in the bare ground treatment.