The contribution of mineralization to grassland N uptake on peatland soils with anthropogenic A horizons

Peatland soils contain large amounts of nitrogen (N) in the soil and mineralization can contribute substantially to the annual mineral N supply of grasslands. We investigated the contribution of N mineralization from peat with respect to the total annual N uptake on grasslands with anthropogenic A horizons and submerged tile drains. The study included i) a pot experiment to determine potential N mineralization from the topsoil and the subsoil, ii) a 1-year field experiment to study herbage yields and N uptake under fertilized and non-fertilized conditions and iii) a 3-year field study where herbage yield and N uptake from the top 30 cm and the entire soil profile were monitored. The 3-year field study yielded an average N uptake of 342 kgha(-1) under non-fertilized conditions but the contribution of subsoil peat N mineralization to the total N uptake was found to be negligible. Our calculations demonstrate that peat N mineralization contributed only 10% to 30% to the total N-uptake, mainly coming from the top 30 cm. Most of the N uptake under unfertilized conditions appears to be largely the result of mineralization from long-term inputs of dung, ditch sludge, farmyard manure, cow slurry and non-harvested herbage

The influence of feeding strategy on growth and rejection of herbage around dung pats and their decomposition

Fresh cattle dung from four farms with different feeding strategies was used to create artificial dung pats in a continuously grazed pasture in order to compare the rejection of herbage growing around the pats, the effect on undisturbed herbage growth under cages and pat decomposition. The first farm was an extensive organic farm (ORGE) with young steers grazing on a biodiverse sward. The second was an intensive organic farm (ORGI) with dairy cattle grazing on a grass/clover sward during the day and fed low-protein forages indoors. The third dung used was from an integrated farm (TNT), where the feeding strategy was aiming for high dung quality by including straw in the diet. The fourth examined dung was from a conventional farm (CONV) aiming for a high milk production per cow, where fertilized grazed grass was the main component of the diet. A human smell test was performed to rank the odour of the four dungs. After 6 weeks of continuous grazing with dairy cattle, herbage yield around TNT pats tended to be lowest, whilst undisturbed herbage yield in and around caged TNT pats was highest (

Leaf CO2 assimilation and leaf dynamics in catch crops during autumn and winter at two levels of nitrogen supply

This study relates the leaf CO2 assimilation and leaf dynamics of nitrogen catch crops to environmental conditions. Winter rye (Secale cereale) and fodder radish (Raphanus sativus) were grown as catch crops in an outdoor pot experiment at two rates of nitrogen supply (N2 higher than N1) in Wageningen, Netherlands, from August 1993 until April 1994. Biomass increased with increasing N supply. There was no net growth after mid-November. The number of tillers in rye and of appeared leaves in radish were higher in N2 than in N1. Leaf appearance rate increased with temperature in fodder radish and rye. Tillering in rye ceased in mid-October. Leaf lifespan was related to the temperature sum between leaf emergence and leaf death. Leaf lifespan was 478 +or- 68 degrees C day in fodder radish. In rye, the leaf lifespan gradually decreased from 592 +or- 66 to 389 +or- 25 degrees C day and from 545 (1 observation) to 401 +or- 64 degrees C day in N1 and N2, respectively. In young leaves, Amax (light-saturated CO2 assimilation rate) was approximately 1.2 mg CO2 m-2 leaf s-1 in September and 0.5 mg CO2 m-2 leaf s-1 later on, independent of species and N supply, N supply affected the organic N concentrations only in the older leaves. Amax was not dependent on temperature at measurement (range: 12-19 degrees C in September, 5-15 degrees C from November until March), but, in contrast, strongly related to temperature and irradiance during the preceding growth period. Leaf nitrate concentrations increased with N supply. Water-soluble carbohydrate concentrations were higher in N1 than in N2 and higher in rye than in fodder radish. They fluctuated during the season. It is concluded that overall growth rates were limited by process rates other than that of leaf CO2 assimilation per unit leaf area in both N1 and N2

Productivity of grasslands under continuous and rotational grazing

In the Netherlands, rotational grazing, with grazing periods of 2 to 5 days, is the most common grazing system at present. In contrast with other countries of North-western Europe, the continuous grazing system is used here only to a limited extent. However, the results of numerous comparative trials at high nitrogen fertilization levels and high stocking rates, carried out in the 1970's, suggest that there is no significant difference in animal production between the two grazing systems.Experiments were carried out to determine the physiological and environmental limits to herbage production under continuous and rotational grazing. This was done by measuring the seasonal patterns and seasonal totals of sward CO 2 assimilation and animal production. The experiments were carried out on heavy clay soils at two nitrogen fertilization levels (125 and about 450 kg N ha -1yr -1). With 450 kg N ha -1yr -1cumulative gross assimilation over the grazing season was 9% higher with rotational than with continuous grazing, but there was no difference in animal production. The higher efficiency of utilization of gross assimilation products under continuous grazing was due to lower topping losses and lower costs of above-ground maintenance respiration. Under both grazing systems, gross CO 2 assimilation per unit leaf area was not depressed at all at 125 kg N ha -1yr -1. but there was a marked reduction of the rate of leaf area development in the second half of the grazing season. The absence of any effect in the first half of the grazing season was due probably to a residual effect of previously applied nitrogen. This effect can be considerable on heavy clay soils.The herbage intake under rotational grazing is often estimated using Linehan's formula, which takes into account the herbage production during grazing. This formula was evaluated by means of dynamic simulation, with measured assimilation-light response curves as the main input. It emerged that in some practical situations of rotational grazing, the herbage production during the grazing period is significantly underestimated using Linehan's formula. This is mainly because Linehan's formula assumes exponential growth of the sward at all stages of growth. Since this is not correct for a sward in the absence of grazing, a new comprehensive formula is derived, using the assumption that the sward is in the linear growth phase at the start of grazing. Comparisons with the simulation output show that this new formula for estimating herbage intake is valid for all situations of rotational grazing.To manage the continuous grazing system successfully, information is needed about the seasonal changes in production capacity with this grazing system. An experiment with dairy cows revealed that throughout the grazing season a constant proportion (equal to 0.25 at the high nitrogen level) of the carbohydrate pool, derived from gross assimilation minus above-ground maintenance respiration, was ingested by the grazing cattle. This observation formed the basis for a dynamic model to compute the net herbage production under continuous grazing throughout the grazing season, using data on radiation and temperature. It appears that, although there is a gradual decline from spring to autumn, the highest production rates occur in June. The average seasonal pattern of net herbage production, predicted by the model using average weather data, can be used for management purposes of the continuous grazing system. It is then possible to adjust stocking density during the grazing season, to achieve the maximum pasture output without adversely affecting the botanical composition and tiller density of the sward. Although the limited time available for this study made it impossible to examine effects of rotational and continuous grazing on the long term, there are good indications that continuous grazing is preferable in this respect. Moreover, the continuous grazing system seems better able to withstand periods with low rainfall, owing to the higher tiller density. During prolonged periods of water stress, however, the mean sward height must be lowered to about 6 cm, instead of the normal optimum level of 7 to 8 cm. It was observed on the clay soils that there is then actually no need for irrigation to achieve high pasture outputs. This is due partly to the unchanged carbon allocation pattern during periods of water stress.The simulated production rates under continuous grazing were compared with measured production rates of grass swards with an initial herbage mass close to the recommended grazing stage for rotational grazing (1700 kg DM haha -1above 4 cm), in order to develop a simple method for estimating the undisturbed herbage production rate at the onset of a rotational grazing period. The herbage production of a sward in the grazing stage for rotational grazing was, on average, 2 or 2.5 times the production level under continuous grazing. The higher value was found for the period early May-early June and was a result of stem elongation. This observation may serve as a simple method for estimation of the undisturbed production rate.A new procedure was developed to deal with grazing losses. Here, the utilization efficiency is calculated by comparing the total amount of harvested dry matter (herbage intake plus silage grass) with the total production under a certain cutting regime. This approach is applicable in any grassland management situation.The CO 2 assimilation of a leaf canopy is strongly dependent on the ratio between diffuse and direct radiation. This produces much of the scattering often observed in field measurements on the assimilation-light response of crop surfaces. It is shown that the crop production models PHOTON and BACROS, developed at the Department of Theoretical Production Ecology of the Wageningen Agricultural University, treat the distribution of diffuse and direct radiation over the leaves of a canopy correctly, but that the proportion of the diffuse component is significantly underestimated over a wide range of radiation levels. Radiation measurements were used to improve the section in these models describing the separation between diffuse and direct radiation. Literature survey showed that the leaf assimilation-light response in field-grown grass swards can be best calculated using a Blackman curve, rather than the frequently used asymptotic exponential curve. With these improvements, the relation between daily total radiation and daily total gross assimilation (and dry matter production) is shown to approximate to a Blackman curve, with the intersection at 60% of the maximum radiation total on that day, i.e. that under a perfectly clear sky