Modelling Soil Organic Matter Dynamics – Global Challenges - Sustainable Management of Soil Organic Matter - page 59

G.M. Richteret al. ModellingVariation in C and N Loss

2.3

Modelling Variation in C and
N Loss and Effects on N
Mineralization After Grassland
Ploughing Over a Catchment

G.M. R ICHTER , G. T UCK AND J.U. S MITH

Soil Science Department,IACR-Rothamsted,Harpenden,
Hertfordshire AL5 2JQ, UK

Introduction

Ploughing grassland has a great impact on organic carbon (C org ) and
nitrogen (N t ) turnover, and related budgets vary considerably. Expressed as
relative total N content, its decline after grassland ploughing can be
approximated by a unique exponential decay function. As a generalization,
50-60% of initial nitrogen will remain under long-term arable land use
(Voroney et al ., 1981; Johnston et al ., 1994) for loamy grassland soils. Diffi-
culties arise for regional quantitative analysis, since long-term observations
are missing, and initial values are often unknown on the landscape scale.
'Snap-shots' of the current status for soil organic matter (SOM) combine
spatial and temporal variation reflecting physicochemical properties of soils
and differences in management. Variable soil physical and hydrological
properties may have caused enrichment and differences in the chemistry of
SOM. One question is whether equilibrium C and N contents of arable
soils will be reached within a period meaningful for farm management (a
decade or a generation). Models, which describe the underlying dynamics
as a function of soil, climate and management variables, may be able to
explain the observed variability. As a first step, we attempt here to simulate
SOM dynamics following ploughing up of grassland using a standard arable
model (SUNDIAL) with modifications only to the soil carbon parameters
as measured. These early predictions will be compared with SOM N
fractions and N mineralization measurements.