1. Introduction
Soil nutrient availability has been largely studied in many different crops and grasslands. It has been shown that many different factors affect it, ranging from climate to soil microorganisms. In the specific case of mountain meadows, there is not much information about nutrient availability, and even less from the management point of view [
1,
2].
Mountain meadows of the Spanish central Pyrenees are grazed during the spring and autumn; there is also mowing at the end of spring or the start of the summer to obtain hay for the winter [
3,
4]. In some cases, and depending on the climate, it is possible to have a second cut. Fertilization can vary according to location and farm management dynamics. Usually, meadows are fertilized using the manure from the farm, or they are not fertilized. We can also find meadows where inorganic fertilizers are applied to increase their productivity [
5]. Typically, because of mountain orography, meadows are in small plots, with steep slopes that complicate their management. Also, their soils are usually shallow and have low fertility, with a climate that makes the growing season short, resulting in low productivity. As farms are getting larger and more mechanized, some meadows have been abandoned, and others have been intensified. In some valleys, up to 40% of meadow areas have been lost [
6,
7].
Meadows are composed of various species from different botanical families, making them important reservoirs of biodiversity [
8]. Although plant diversity makes them more adaptable to a various range of conditions, it makes them more complex to study, as each meadow will behave differently, according to its floristic composition [
9].
Management has an impact on different aspects of meadow dynamics. Mowing frequency, fertilization and livestock load can change floristic composition and nutrient availability [
10,
11]:
- -
Increasing the livestock load will increase the amount of nutrient input as excrements, but on the other hand can put excessive pressure on plants and provoke changes in meadow composition [
12].
- -
Fertilization has been shown to be one of the major drivers of floristic composition changes, especially when there is inorganic nitrogen fertilization, which can lead to an increase in grass species and a decline in legumes [
10,
13,
14]. Nutrient availability is also affected by changes in management. For example, when too much nitrogen is applied, it could lead to depletion of the phosphorus and potassium soil reservoir and therefore reduce yield [
15,
16].
- -
Even the fertilizer type can have an impact. Organic fertilizers are usually applied in high volumes and have much organic matter, enhancing nutrient availability and enrichment of soil, whereas inorganic fertilizers can also enrich the soil, but nutrients could remain immobilized [
17,
18].
The nutrient balance of meadows has been poorly analysed compared to other crops, even though it determines meadows’ long-term sustainability. Nitrogen, phosphorus and potassium are the most important nutrients; if the balance becomes negative year on year, it will cause nutrient depletion and consequently yield decrease [
19]. On the other hand, when the balance is positive year on year, especially in the case of nitrogen, it could lead to environmental issues like water eutrophication. In addition, a positive balance of any nutrient implies that the farmer is investing more resources than needed to get the same yield, making their business less efficient and affecting their profitability [
15,
20].
There are many environmental ecosystemic services that are provided by meadows, such as serving as biodiversity reservoirs, decreasing fire risks, and reducing erosion [
21]. Carbon storage is also an important ecosystemic service, as meadows act as carbon sinks through the accumulation of organic matter. As there is no ploughing, organic matter accumulates in the soil, although it has been observed that during heat and drought stress, meadows can swift from carbon sinks into carbon sources during the stress [
22,
23,
24,
25].
In this study, we are going to analyse the nitrogen, phosphorus, potassium and carbon balances of three different meadow types, which have three different levels of intensification practices. Our hypothesis is that meadow intensification could increase nitrogen soil concentration, which could cause environmental problems such as water eutrophication due to nitrogen lixiviation. On the other hand, we expect phosphorus and potassium soil concentration to decrease as intensification increases, because of fertilization imbalance. We also expect to find higher carbon accumulation in extensive meadows.
2. Materials and Methods
The study is in the central area of the Spanish Pyrenees, in the valleys of the Gállego, Cinca and Ésera rivers. Along the three valleys, we sampled a total of 12 meadows corresponding to 3 different types: intensive, semi-extensive, and extensive [
26,
27]:
- -
Intensive meadows: located in the bottom of the valley, usually close to the barn, where the animals stay many days, resulting in high livestock loads. To be able to deal with the livestock load and increase their productivity, the meadows are usually fertilized with inorganic fertilizer.
- -
Semi-extensive meadows: located in the middle of the valley, also close to the barn and usually in deep soils with high fertility. Due to their location close to the barn, they are fertilized with manure from the farm, which increases their productivity and soil fertility. But as the climate is colder than in the bottom of the valley, there are fewer available days for grazing, resulting in lower livestock loads.
- -
Extensive meadows: located in the high areas of the valley and surrounded by forest, they are the fields farthest from the farm. They are rarely fertilized, mainly due to the distance from the farm, and as a result they have lower productivity. Also, the colder climate limits the number of days where they can be grazed, which in combination with their lower productivity leads to lower livestock load capacity.
We can observe in
Figure 1 that intensive meadows are in the lower parts of the valleys, and as a result they are at lower altitudes. In
Table 1, we found the highest altitudes in extensive meadows. All meadows except extensive meadows are fertilized yearly. The fertilizer type is organic, except in intensive meadows, where it is inorganic. Slopes are higher in semi-extensive and extensive meadows. Legume cover is similar in all cases, being more variable in intensive meadows. All meadows are in Haplic Regosol soils. Intensive meadows have more clay content, whereas extensive meadows have more sand. pH is higher in intensive meadows. Conductivity is higher in semi-extensive meadows. Organic matter in extensive and semi-extensive meadows is more than double that of intensive meadows.
Hay samples were taken from 6 plot enclosures (40 cm × 60 cm), half of them being mowed as the same time as the farmer and the other half earlier, when the crop reached its optimal protein content. Production (kg DM/ha) was calculated by weighing the oven-dried biomass (65 °C for 48 h).
For nutrient extraction, protein and fibre content, samples were taken to the laboratory, where the determinations were performed as follows: laboratory dry matter at 105 °C for 4 h; nitrogen content (N) was determined using the Kjeldahl method, and crude protein (CP) concentrations were calculated from it by multiplying N × 6.25. Ash-free neutral detergent fibre (NDF) and acid detergent fibre (ADF) were quantified using an Ankom 200 fibre analyser (Ankom Technol. Corp., Fairport, NY, USA), according to Van Soest et al. [
28].
Relative feed value (RFV) is an index that combines important nutritional factors (potential intake and digestibility) into a single number, providing a quick and effective method for evaluating feed value or quality. The RFV was calculated using the estimates of digestible dry matter (DDM%) and potential dry matter intake (DMI% of body weight) of the forage, based on the ADF and the NDF fractions [
29].
Soil samples of 1 kg were taken at random from 3 different places in the meadows using an Eijkelkamp hand auger and homogenized into a single sample. They were taken at two times: at the beginning of the year, during February, before fertilizer was applied, and the last one in June, right after hay harvest.
Soil analysis covered nitrogen in its nitrate fraction by spectrophotometry analysis, phosphorus by the Olsen method, potassium by ammonic acetate extraction, and organic matter by spectrophotometry.
The Braun-Blanquet method [
30] was used to carried out flora inventories in the central 100 m
2 of each meadow. From them, the Shannon index was used to calculate plant biodiversity (35).
The climatic parameters were obtained from the climatic stations 9446, 9789A, 9784P, 9814X, 9838A, 9838B, and 9843A of the network of the Agencia Estatal de Meteorología [
31]. The accumulated rainfall and the accumulated evapotranspiration (ETO) were calculated daily from March 1st using the Penman-Monteith method. Evapotranspiration was calculated from temperature data according to CROPWAT 8.0 procedures [
32]. The growing degree days were calculated 1 February 1st by the sum of the daily average temperatures when they were between 0 and 18 °C. If the averages were negative, we limited them to 0 °C, and to 18 °C when they were > 18 °C [
33,
34]. All climate parameters were calculated until the mowing day.
Farm information was obtained from interviews with the farmers where they were asked how much time the meadow is grazed, the number of animals, how much fertilizer is applied and what type, and what is the average production of the meadow. Stocking rate was expressed as livestock units (LU) per ha.
As was shown in [
27], spring grazing corresponds to 15% of the hay production, whereas autumn grazing is 25% of the hay production. In order to get the nutrient extraction by grazing, we use the following equation:
where G is nutrient extraction by grazing, M is nutrient extraction by mowing, K
a is a coefficient that indicates if the meadow was grazed in autumn (1 if grazed, 0 if not) and K
s is a coefficient that indicates if the meadow was grazed in spring (1 if grazed, 0 if not).
Nutrient input via excrements were estimated using stocking load, and data from previous studies were nutrient concentrations of livestock excrements were determined [
35], as shown in the following equation:
where E is nutrient input by excrements, N is the number of days spent by the animal in the meadow per hectare, and C is the daily nutrient input according to the literature, as it can be summarized in
Table 2:
Nitrogen fixation was estimated according to meadow legume cover and using data from previous studies, which calculated 160 kg ha
−1 for Pyrenees meadow legumes [
27]. The following equation was used:
where NX is nitrogen fixation by legumes in kg ha
−1, and LC is legume cover in m
2 of legume surface per m
−2 of crop surface.
Nutrient crop balance was calculated by adding the values of nitrogen fertilization and nitrogen content estimated in animal droppings and subtracting nutrient extraction by mowing and estimated nutrient extraction by grazing, as shown in the following equations:
where NB is nitrogen balance, NF is nitrogen fertilization, NE is nitrogen in excrements, NX is nitrogen fixation, NE is nitrogen extracted by mowing, and NG is nitrogen extracted by grazing.
where PB is phosphorus balance, PF is phosphorus fertilization, PE is phosphorus in excrements, PM is phosphorus extracted by mowing, and PG is phosphorus extracted by grazing.
where KB is potassium balance, KF is potassium fertilization, KE is potassium in excrements, KM is potassium extracted by mowing, and KG is potassium extracted by grazing.
Soil organic carbon (SOC) was calculated using the following formula:
where OC is the organic concentration (%), LT is the layer thickness (in our case 0.3 m), BD is the bulk density (Mg m
−3), and RF is the rock fragment concentration [
36].
To calculate soil balance, nutrient soil concentration in June was subtracted by nutrient concentration in February, as shown in the following equation:
where SB is soil nutrient balance, N
j is soil nutrient content in June, and N
f is soil nutrient content in February.
All data were checked by the Shapiro–Wilk normality test, and as the data were nonparametric, the Wilcoxon test was used to compare differences among meadow type and year. Additionally, the post hoc Wilcoxon test was carried out to check between which group were the differences.
We have considered the differences to be statistically significant when
p < 0.05. All statistical analyses were carried out using R version 4.4.0 [
37].
5. Conclusions
Meadows are complex systems where management practices have implications for soil nutrient availability.
Nitrogen availability depends both on management and climate. Inorganic fertilization is the least stable source, decreasing fast in the soil when there are more rainfall events; on the other hand, semi-extensive meadows have higher soil nitrogen stability even though it is applied in higher doses. Extensive meadows, despite lacking nitrogen fertilization, are more stable, relying less on climate.
Usually, phosphorus is the nutrient that limits yield, as is clearly shown in extensive meadows, which have reached an equilibrium between input and output. In intensive and semi-extensive meadows, we found lower phosphorus accumulation than the crop balance estimates, probably due to its blockage.
Potassium is clearly influenced by temperature, as we have found high differences between its concentration in June versus its concentration in February, even when the crop balance shows negative values. Only in extensive meadows is there always a potassium deficit, which could eventually cause potassium deficiency.
Carbon accumulation is also highly influenced by climate. Our results showed that when there is drought stress, meadows shift from a carbon sink into a carbon source. Extensive meadows were the only type that was able to accumulate carbon even in the drought year.
Further research will be needed to assess the long-term consequences of management, as this study is based on only two specific years.