Saturday, 15 July 2006

The PO4/NO3 Ratio in Seepage Waters as an Indicator for the Macropore Contribution to Leaching.

Martin Kuecke, Deok Hoon Yoon, and Joerg-Michael Greef. Institute of Crop and Grassland Science, Federal Agricultural Research Center, Bundesallee 50, Braunschweig, 38116, Germany

Hypothesis: Leaching intensity and dynamics are influenced by cropping practices (soil cultivation, crop rotations, fertilization), soil conditions (texture, soil moiture, soil structure, ground water level) and climate (rainfall intensity and distribution over the seasons). Under specific conditions, macropore flux can contribute significantly to leaching (e.g. heavy soil, sity with zero or minimum tillage, situations with heavy rainfall events in dry dry seasons and on dry soils). As the conditions for macropore flow alter over the time, the ratio of macropore flow/matrix flow is not a constant. The major positive effect of macropore flow is the improved drainage of excees water resulting in decreased soil erosion and surface runoff, while a negative effect is the downward transport of nutrients and pesticides that are not leached through the soil matrix (e.g. phosphates). There is no way to find out in field studies to which extent macropore transport contributes to leaching. It has been shown in some published field studies that nitrate is leached out of soil profiles by matrix water flow, while phosphate is transported downwards merely through macropores from the soil surface. Based on these findings, it was the hypothesis of this study that ratios of nutrients that are preferentially transported through macropores with those preferentially transported through the soil matix can be used as an indicator to identify seasons and situations with a significant macropore transport contribution. Between 2001 to 2004 the leaching of mineral and organic nitrogen and phosphorus was studied in long-term field experiments (elder than 6 years before the start of this study) on 2 different soil types (sand, clay loam) with different soil cultivation (conventional tillage, minimum tillage), land-use (grassland, arable-land) and fertilization (exclusive organic and mineral fertilization). Passive capillary samplers (PCAPS, fiberglass wick sampler) were chosen to collect seepage water. Four PCPAS were installed below the undisturbed soil in each plot via trench walls and tunnels 80 cm below the soil surface. PCAPS are installed below the undisturbed soil profile and collect matrix flux as well as macropore flux. As the chemical interaction between the fibreglass and the compounds in the seepage is low, the determination of nitrogen and phosphorus leaching under undisturbed soil conditions over seasons and years is possible. Seepage waters were collected weekly, and NO3-, NH4, total N, PO4 and total P concentrations were measured in the samples, organic N and P were calculated from the difference of total-N (P) minus min. N (P). The highest concentrations of mineral N (NO3- and NH4) were detected generally in the late winter, while mineral P concentrations were highest in late summer. Organic N and P concentration did not show any clear seasonal trends. The average share of ammonium-N in total nitrogen was 15%. Twenty-two percent of the total nitrogen was found as organic N in the drainage from the clay loam (for both conventional and minimum tillage) in average of the monitoring period, while it was only 7% for the sand soil, both grassland and arable-land. More organic P was found in the leachates from the sandy soil (47% of total P). On the clay loam soil, the contribution was 38% on the minimum tillage plot and 15% on the conventional tillage plot. It is noticeable that the organic N concentrations in the seepage waters decreased in most cases when the organic P concentrations increased, and vice versa. In most cases, a decrease of the nitrate concentration was parallel observed with an increase of the PO4 concentration. This confirms the hypothesis of this study. In such situations, a contribution of macropore/preferential flow to leaching is assumed. The PO4/NO3 ratio was highest on all sites the late summer months. It is concluded that the P/NO3- ratio can be used as a suitable indicator to identify the presence of macropore flow and transport.

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