The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 214

This chapter discusses some further evidence for the importance of
good organic matter management in maintaining nutrient supplies, and the
conflict between this and minimizing losses to the environment. It does
this by moving progressively through three areas of research that currently
are of great international interest and which dominate the chapters in this
part of the topic. First, it reviews developments in the measurement of gross
N transformations, especially mineralization; such measurements enable
the release of N from the breakdown of SOM, crop residues and manures to
be better studied, and highlight the central role of carbon (C) in N cycling.
Secondly, it considers the size and significance of the soluble organic N
pool in soils. This pool is proving to be central to rapid C and N turnover,
but also a potential organic source of N loss to the environment. Thirdly, it
reviews some recent research showing the greater effectiveness of manures
over fertilizers for supplying phosphorus (P) and potassium (K).

Gross N Transformations

If N is to be used efficiently in agroecosystems, the factors and mechanisms
that influence its supply must be understood. In the past, it was difficult to
determine the relative importance of key processes in N turnover -
ammonification, nitrification and immobilization - because they could
not be separated. Isotopic pool dilution with 15 N enables gross rates of
N transformations to be determined (Powlson and Barraclough, 1992).
Measurements made on soils from a range of land uses at Rothamsted and
elsewhere show how these techniques are identifying key processes and
linking process rates to land use, management and other controlling factors,
especially C (Murphy et al ., 1998, 1999a).
Soils were collected from a plot of the Broadbalk Experiment, which
has received no fertilizer since its inception in 1843 and has been under a
5-year rotation of fallow, potatoes, wheat, wheat, wheat since 1986, and
from the unlimed, unfertilized plot (3) of the Park Grass Continuous Hay
Experiment. Soil samples were taken from each plot, sieved to < 5 mm,
bulked to give a representative sample, and stored at 4 C in the dark

until used. Isotopic pool dilution experiments were conducted on three
replicates from each bulk sample, as described by Willison et al . (1998),
to measure gross rates of ammonification, ammonium consumption and
nitrification (Fig. 4.1). Ammonium immobilization was estimated as the
difference between ammonium consumption and gross nitrification.
Net rates of mineralization for the two experiments are of the same
order of magnitude: we measured them to be 0.13 and 0.55 mg N kg −1
day −1 for Broadbalk and Park Grass, respectively. However, gross N
transformation rates (between arrows in Fig. 4.1) and pool sizes (in boxes
in Fig. 4.1) are strongly affected by land use and differ by at least one order

°