The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 231

F.Vertèset al. Nitrogen Mineralization Under Bare Soils

4.2

Nitrogen Mineralization
Under Bare Soils After the
Destruction of Grazed Pastures

F. V ERTÈS 1 , F. L AURENT 2 , S. R ECOUS 3 , P. L ETERME 1
AND B. M ARY 3

1
INRAUSARQ, 4 rueStangVihan, F-29000 Quimper; 2 ITCF,
F-91720Boigneville; and 3 INRAAgronomie, F-02007Laon, France

Introduction

The proportion of herbage grazed by cows in intensive livestock systems in
Brittany (France) currently is increasing, as a result of the reduced cost
of production (compared with maize plus concentrate-based systems).
Moreover, the frequency of mixed grass-clover pasture is increasing, in
order to reduce N fertilizer applications and to obtain a better distribution
of production throughout the season (particularly in summer). Most of
these grasslands are regularly destroyed (after 4-7 years) to grow silage
maize or winter wheat, releasing large amounts of nitrogen into the soil and
thus increasing the risk of nitrate leaching. This risk must be taken into
account when comparing the sustainability of different systems. The work
presented here had two aims: (i) to determine the amount and kinetics
of nitrogen mineralization following the destruction of grassland; and
(ii) to determine the factors which affect mineralization, i.e. previous N
management (fertilization rate and timing), type of grassland (pure grass or
grass-clover) and soil organic matter content.

Materials and Methods

The experiments were conducted on two sites in Brittany during the period
1990-1998. The first site, Kerlavic, was a well-drained sandy loam soil
of 0.8 m depth, with 6.5% organic matter content and a pH of 5.8.
Four

treatments were set up: one was clover-grass sward receiving