『Abstract
In low input farming systems the reserves of organic and inorganic
P may be a significant source of P to crops. To clarify this we
investigated organic and inorganic P reserves in an array of irrigated
and rain-fed calcareous soils of grassy crops, with long term
organic or conventional farming practices. Organic P (Po)
reserves appeared to be more sensitive than inorganic P reserves
to changes in soil fertilisation regime. Organic P reserves were
sensitive to the quantity of P added, rather than to the quality
of the input. In soils receiving manures, there was nocorrelation
between organic P reserves and soil organic C and N or soil carbonate
content. In contrast, inorganic P reserves in soils receiving
manure were positively linked to soil total N and to organic C
(in irrigated organic soils) and negatively to carbonate content.
This suggests that the accumulation of inorganic P pools in calcareous
soils receiving manure may be enhanced in soils with high organic
matter content. As indicated by changes in the N/Po
ratio, soil carbonates protected total organic P pools from mineralisation
in soils receiving adequate amounts of P fertilisers. In contrast,
in soils receiving low amounts of manures we observed low amounts
of organic P even in soils with a high carbonate content. Therefore,
in conditions of P starvation, organic P reserves protected by
carbonates may be slowly mobilised. Despite this mobilisation,
in soils receiving low amounts of P, no relationships were observed
between labile Pi pools and organic P reserves.
In contrast in soils in soils receiving sufficient amounts of
P we observed a relationship between organic P reserves and labile
Pi pools. In irrigated soils inorganic P
reserves were positively correlated with labile P reserves and
labile Pi pools. In irrigated soils inorganic
P reserves were positively correlated with labile P pools, even
when fertilisation was moderately low. The application of manure
to these latter soils favoured the links between organic P reserves
and labile P forms.
Keywords: Soil fertility; Soil mining; P balances; Use of manures;
Low input farming systems; N/Po ratio』
1. Introduction
2. Material and methods
2.1. Study area and management practices
2.2. Experimental design and soil sampling
2.3. Soil analysis
2.4. Statistical analyses
3. Results
3.1. Soil P reserves
3.2. Soil properties and P reserve
3.3. Relationships between labile P forms and P reserves
4. Discussion
4.1. Farming practices and soil P reserves
4.2. Regulation of P reserves
4.3. Contribution of P reserves to the availability of P
4.4. Pathways and factors of regulation
5. Conclusions
Acknowledgements
References