Abstract

There is increasing pressure in the UK to adopt alternative, sustainable uses for agricultural land. New schemes indicate a trend towards lowland afforestation schemes which are a very new concept in Britain. One such scheme is agroforestry whereby trees and a crop are grown together on the same unit of land. Very little is known about the impact of agroforestry on soil hydrology and soil water nitrate distribution of trees planted at a regular spacing into grazed grassland. In this case a lowland grazed silvopastoral agroforestry system in Devon, UK, was monitored to determine the impact upon the soil by the trees planted at a regular spacing. An innovative detailed programme of monitoring was conducted as a baseline study using six year old ash trees (Fraxinus excelsior) planted at 400 stems per hectare into perennial ryegrass pasture, from 1993 to 1994. Comparisons were made with adjacent grazed grassland and farm forestry sites. The above and below ground hydrology and fluxes of soil water nitrate were examined for temporal and spatial patterns on a seasonal and storm basis. In addition detailed measurements on soil water nitrate distribution were investigated on a storm and pulse basis. The main findings of the research were that interception by the trees was minimal and stemflow accounted for 1% of precipitation. Overland flow was a dominant flow process. In winter approximately 70% of net precipitation flowed overland. Soils were at or above field capacity from September 1993 until May 1994 and infiltration rates were low at about 28 - 35 mm dayˉ¹. The soil saturated rapidly from 300 mm depth during the winter indicating a lack of storage. There was no difference in quantities of overland flow between the agroforestry and the grassland treatments, but storm hydrographs were affected slightly. Flows along root surfaces or old root channels conducted the water rapidly to 300 mm depth. Considerable amounts of nitrate (35%) were lost by overland flow in April and May 1993. There was no significant difference of nitrate in either overland flow or soil water during storms. A tracer experiment conducted in May I 994 showed the presence of trees in grazed pastures influenced the dynamics of soil water movement and transport of nitrate. As the trees grow further modifications to the soil and wider environment will be inevitable.

Document Type

Thesis

Publication Date

1997

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