RESULTS AND DISCUSSION

INTRODUCTION AND OBJECTIVES The P source factor is an essential component of the P index, which is a measure of the risk of

P loss from land to surface waters (Sharpley et al., 2008). The P source factor is usually

estimated from the concentrations of P measured in soil, taking into account the inputs of P to

the soil via organic and inorganic fertilizers and also the method of application of these

fertilizers (Whiters et al., 2003). Obviously, the value will vary in fertilized soils because it will

be strongly affected by fertilizer application and by plant growth cycles (Beegle et al., 2006).

However, little is known about how the P source factor varies throughout the agricultural year in

cropped land, where large amounts of P may be transported from the soil to surface waters.

We investigated the variation in the P source factor during the period of maximal plant growth

(March-October) in the Fonte Espiño-Rego de Abellas basin, Galicia, NW Spain (Fig. 1).

MATERIALS AND METHODS The basin covers an area of 772 ha and the main land uses are grassland (both natural and

planted with a mixture of graminaceous and leguminous plants) and forest (reforestation with

Eucalyptus globulus Labill and Pinus radiata D. Don.), with some small agricultural plots used for

domestic purposes. Between March and October, soil samples were obtained fortnightly from 36

sampling points in the catchment (forest and grassland soils, Fig. 1). Each sampling point (ca. 0.5

ha) was selected beforehand and considered representative of different areas. Composite

samples comprising 10–15 individual subsamples were collected from the upper 0 to 5 cm of the

soil after removing plant remains from the soil surface. The samples were analysed for general

soil properties and the P source factor was considered as the inorganic P extractable with 0.5 M

NaHCO3, pH 8.2, according to Olsen et al. (Guitián and Carballas, 1976).

Fig. 2. Monthly maps of the P source factor in the Monte Espiño – Rego de Abella basin.

TEMPORAL VARIATION IN THE P SOURCE IN A SMALL GRASSLAND

WATERSHED F. Gil-Sotres1, F. Troitiño1,2, M.C. Leirós1, C. Trasar-Cepeda2

1 Dpto. Edafología y Química Agrícola, Fac. Farmacia, USC, Grupo Evaluación Calidad del Suelo, U. Asociada CSIC, 15782 Santiago, Spain 2 Dpto. Bioquímica del Suelo, IIAG-CSIC, 15780 Santiago de Compostela, Spain

Fonte Espiño–Rego de Abella system

Xallas Basin

Xallas Basin

Basin under study

Xallas River

Xallas BasinXallas BasinXallas BasinXallas Basin

Xallas Basin

Basin under study

Xallas River

Xallas Basin

Basin under study

Xallas River

Xallas BasinXallas Basin

Basin under studyBasin under study

Xallas RiverXallas River

1

8

7

6

5

4

3

2

1

8

7

6

5

4

3

2

Fonte Espiño–Rego de Abella system

Xallas Basin

Xallas Basin

Basin under study

Xallas River

Xallas BasinXallas BasinXallas BasinXallas Basin

Xallas Basin

Basin under study

Xallas River

Xallas Basin

Basin under study

Xallas River

Xallas BasinXallas Basin

Basin under studyBasin under study

Xallas RiverXallas River

1

8

7

6

5

4

3

2

1

8

7

6

5

4

3

2

Throughout the study period, the values of the P source factor varied little in the forest soils

but varied widely in grassland soils (Table 2). In general, the highest values were obtained in

July and August, while the lower values were obtained at the beginning of the spring (March-

April).

The soil scores varied from low or very low to high. Elaboration of P source factor maps enabled

us to distinguish areas where the P source factor remained constant throughout the study period

from areas where the factor varied widely (Fig. 3). Maximun risk of soil P losses occurs from

the second fortnight of May until the end of August.

In forest areas, the P source factor remains almost constant throughout the year.

In cultivated areas, and because of the soil management, the P source factor is variable

throughout the crops growing period.

Frequent and isolated fertilizations, like those carried out in agricultural areas dedicated

to grassland, difficult the estimation of the P index, because cause the spatial variation

of critical P loss areas throughout the year.

CONCLUSIONS Beegle, D.B. Weld, J. L., Gburek, W. J., Kleinman, P. J. A., Sharpley, A. N., Kogelmann, C., 2006. Extension Pub. CAT UC180, University Park, Pennsylvania, USA. Guitián, F., Carballas, T., 1976. Técnicas de Análisis de Suelos. Ed. Pico Sacro, Santiago de Compostela, Spain. Sharpley, A.N., Kleinman, P.J.A., Heathhawaite, L., Gburek, W.L., Weld, J.L., Folmar G.J., 2008. Journal of Environmental Quality 37, 1488–1496. Whiters, P.J.A., Ulen, B., Stamm, C., Bechmann, M., 2003. Journal of Soil Science and Plant Production 166, 459-468.

REFERENCES

ACNOWLEDGEMENTS This study was financed by the BUFFER project (EVK1-CT-1999-00019 European Union Contract). Financial

support was also provided by the Ministerio de Ciencia y Tecnología of Spain (Acción Especial REN2000-1806-

CE/HID) and by the Xunta de Galicia (Incentivo Proyecto Europeo PGIDT00PXI20306PM). The authors thank Ana

I. Iglesias and Isabel Martinez for their assistance in carrying out analyses.

Table 1. Main characteristics of the soils investigated in the study.

Fig. 1. Map of Galicia and location of the Fonte Espiño – Rego de Abella basin.

Table 2. Monthly mean (±s.d.) values and mean values for all period of study (coefficient of variation CV %) for P source factor (mg kg-1) in the soils under the main land uses.

Forest Native grasslands Fertilized grasslands

(n=22) (n=20) (n=30)

March 1.89±1.83 3.95±2.00 21.78±12.46

April 1.15±0.91 3.60±1.92 21.42±11.34

May 2.17±2.00 4.10±1.41 19.48±12.14

June 2.00±0.51 4.28±1.48 21.72±10.29

July 1.75±1.58 6.91±6.14 36.04±19.82

August 2.28±0.78 5.52±3.14 25.88±12.42

September 2.35±0.82 4.13±1.39 20.36±10.68

March-September 1.94±0.82 (42%) 4.64±2.50 (54%) 23.81±15.75 (66%)

Forests Natural grasslands Fertilized grasslands

(n=22) (n=20) (n=30)

Water pH 4.71±0.34 5.29±0.28 5.27±0.30

KCl pH 3.92±0.26 4.41±0.20 4.42±0.25

Total C (%) 9.28±1.84 9.29±1.99 5.94±1.28

Total N (%) 0.74±0.14 0.83±0.14 0.58±0.12

C/N 12.8±1.2 11.1±1.1 10.3±0.5