The nitrate concentration in the upper groundwater in sandy soils is considerably higher under maize than under grass. What role does crop rotation play in this, and how can one reduce nitrate leaching?
Article source: Water Matters
In the Evaluation of the Meststoffenwet (Fertilisers Act) the Netherlands Environmental Assessment Agency (PBL) states that the nitrate standard of 50 mg/l in the groundwater is achieved almost everywhere on average. Only in ‘Sand south’ (sandy soils in Noord Brabant and northern and central Limburg) is the average concentration higher than the nitrate standard. This can partly be explained by the soils in Sand south's greater vulnerability to nitrate leaching. Another cause for exceeding the nitrate standard in Sand south is the greater proportion of maize in this area. Maize is cultivated as a livestock food crop, often in rotation with grass.
Steps have already been taken in order to restrict the leaching under maize land. Hence farmers are required to grow a catch crop after the maize on sandy and loess soils. Farmers who use row fertilisation in maize cultivation can claim a relaxation of the nitrogen application standards. This article examines the effect of rotation on nitrate leaching and explores the possibilities for action in order to reduce the nitrate leaching under maize fields.
In order to determine the effect of crop rotation on the nitrate concentration, we have made use of data from the Minerals Policy Monitoring Programme (Landelijk Meetnet effecten Mestbeleid - LMM, www.rivm.nl/lmm). The LMM is a measuring network that has been used to monitor water quality and operations since 1992. The LMM measures the quality of the water that drains from the roots zone of the agricultural plots annually at 450 commercial farms.
For sandy soils the leaching is measured in the upper metre of groundwater or, if the groundwater is more than 5 metres below ground level, in the soil moisture in the layer between 1.5 and 3.0 metres. Shallow measurement means that the effects of agricultural practices become clear as quickly as possible.
Water is sampled at 16 randomly selected points at each farm. Two mixed samples are made from this, and the water quality of these is determined in the lab using a broad analysis package. The nitrate concentration of the 16 individual samples is measured in the field using a Nitrachek method protocolled by RIVM. The coordinates of the measuring points are linked to crop information derived from the Basic Register of Crop Plots (Basisregistratie Gewaspercelen - BRP). We have the crop history for the plot since 2009 available for each measuring point. This provides a maximum of 6 years of crop rotation prior to measurement, depending on the year of sampling. For this study we have selected nitrate measurements on sandy soils on which grass or maize was grown in the preceding year. This involves measurements between 2010 and 2015 (approximately 10,000 nitrate measurements).
The measured nitrate concentration in the upper groundwater is linked to the crop in the preceding year, since the upper metre of groundwater is made up of the addition to the groundwater from the preceding year. At deeper groundwater levels the transit time through the unsaturated zone results in an even greater delay.
The nitrate concentration under plots where maize was cultivated in the preceding year is around twice as high as under grassland. This is partly the result of the higher denitrification in the turf; nitrate is thereby converted into nitrogen gas. In addition, maize land lies fallow in the autumn for some time (depending on the effectiveness of the catch crop) as a result of which the nitrogen remaining in the soil after harvesting can leach out more easily as nitrate.
The nitrate concentration does not just depend on the crop in the preceding year, but also on the cultivation history over the years prior to that. For first year grass after maize the average nitrate concentration is almost 60 mg/l (Figure 1). As the period when grass is grown after maize lengthens, the nitrate concentration declines. The turf develops over these years as a result of which denitrification increases and nitrogen is captured in the turf. There is also a ‘lag’ effect; it takes a couple of years before the high nitrate concentration from the maize production has been washed away.
The maximum number of years for which we have crop information is 6 years; for 6 year grass we therefore do not know what was cultivated previously, grass or maize. The nitrate concentration is higher for a period of grass cultivation of 6 years or more than for a shorter period of grass cultivation. We suspect that this is because old grass does not absorb additional nitrogen anymore; the capture of organic nitrogen by the crop is roughly equal to the decomposition. A longer period of grass cultivation also means that there is a higher probability of the grass being ploughed up sometime during the period. A lot of nitrogen is released when grassland is ploughed up, and this leads to a higher average nitrate concentration. Information about the ploughing up of grassland in a grass-grass rotation is not available for the measurement locations.
For first year maize after grass, the nitrate concentration is around 100 mg/l. This nitrate concentration is high because the ploughed up turf releases a lot of nitrogen which is not all absorbed by the maize plants. As the period of maize after grass extends, the nitrate concentration appears to increase. In view of the small number of observations, the confidence intervals are large and this difference was not significant. However, we do see that the nitrate concentration for maize for 6 consecutive years or longer is significantly lower than for a shorter period of maize consecutively. We explain this decline by the fact that the effect of ploughing up grassland has then been exhausted.
Effects of the rotation of grass and maize
In order to study the effect of different rotations on the basis of the information specified above, we have created three imaginary farms on sandy soils with 20% maize land and 80% grassland and varying rotation. Farm 1 has no rotation, farm 2 has 40 percent of the fields in rotation between grass and maize (maize for two years, grass for two years). Farm 3 has 100 percent of the farm in rotation of 4 years grass and 1 year maize (see figure 2)
Farm 1 only has permanent crops. With permanent maize (76 mg/l on average, >=6y in figure 1) and permanent grass (34 mg/l >=6y in figure 1) this farm has an average nitrate concentration of 42 mg/l.
Farm 2 has 60 percent permanent grass, with an average of 34 mg/l, 20 percent young grass (average of first and second-year grass, 47 mg/l) and 20 percent young maize (average 98 mg/l). On average across the entire farm the nitrate concentration is 49 mg/l.
Farm 3 consists of 4 equal parts of grass between 1 and 4 years old (57, 36, 33, 28 mg/l respectively) and one part first year maize (97 mg/l). For this fictitious farm the average nitrate concentration is 51 mg/l.
This shows that the rotation of grass and maize leads to higher leaching of nitrate than permanent cultivation. In order to grow maize, the grassland is ploughed up and a large amount of nitrogen is released. This nitrogen is not immediately absorbed fully by the maize, particularly if no account is taken of this in the fertilising, and leaches as nitrate. In the reverse situation where grass is sown after maize, it takes a couple of years before the nitrate concentration falls again and the turf has built up sufficiently to absorb or break down nitrates.
In the examples we have adopted 20 percent maize throughout. If the percentage of maize is higher, the average leaching at the farm will automatically become higher. This percentage has more of an impact on the average than whether or not grass and maize are rotated.
Based on this data the conclusion that restricting rotation can reduce nitrate leaching appears justified. However, restricting rotation conflicts with advice from agronomists and is not consistent with the results from research at experimental farms.
Cultivating maize for a longer consecutive period can have a unfavourable impact on the level of organic matter in the soil. The soil becomes depleted. In theory this could also lead to greater nitrate leaching. A healthy, fertile soil gives higher yields and therefore less leaching. Nitrates can also be decomposed better in a soil rich in organic material. Crop rotation is also a means of countering pests and diseases.
Research at the De Marke experimental farm shows that nitrate leaching is actually lower when grass and maize are rotated. In practice a benefit can still be achieved by applying the cultivation methods used in this kind of research more widely. The most important advice is not to fertilise first year maize on ploughed up grassland. Every kilogram of nitrogen which is applied then is entirely wasted and will leach to the upper groundwater. Other possible measures that reduce the leaching of nitrate in the event of rotation are:
The wide application of these methods in practice will reduce the gap between practice and experimental farms and the benefit of crop rotation (higher organic material, limited disease) can be combined with reduced leaching of nitrates.