Do large herbivores have an impact on soil quality in Faia Brava Reserve? ?

Faia Brava soils are known to be quite poor in nutrients, minerals and organic content. However, no data were available, since little research had been done so far in Faia Brava. We are missing basic information on soil quality for several reasons. The main question was whether or not the present herbivore management is impacting soil quality. The second question was whether or not we should intervene with regard to reforestation Since 2010 large parts of Faia Brava have been populated with Garrano horses and Marones cows. In Rewilding, herbivore grazing is supposed to have a positive impact on soil quality, since the grazing activities convert raw organic matter into better degradable organic matter i.e. the manure, which in its turn can be food and habitat for worms and insects. In the month of June 2021, ATNatureza carried out an extensive soil investigation on the impact of extensive grazing on soil quality in Faia Brava (Grazelife project from Rewilding Europe). 61 sites were analyzed and at each site 8 soil samples were taken. We compared the following sites:

  • Grazed and non-grazed by herbivores,
  • recently planted trees for forestation,
  • green fertilizers were sowed
  • more moisty sites where water flows in a stream in spring.

Results

The overall result is that the impact of herbivores is negligible on the quality of the soil. This is shown in the following table:

 

% organic matter

St. dev

Carbon (T/ha)

With grazing

2,19

0,38

19,7

Without grazing

2,31

0,39

20,6

We observe that the organic matter in the soil is very low: 2,25% and Carbon in the soil = 1,31%. In areas without grazing, the higher biomass level above the soil, is not reflected in significant higher Carbon% in the soil. We are a bit surprised that also at moisty sites, organic matter was not significantly higher.

A bit as a surprise, the PH was not as low as supposed: 6,35.

The magnesium average in the plots is 87mg/kg (st. dev. = 77,28). However at quite some spots there is a magnesium deficiency, see also the high standard deviation.

The correlation between Boron and Carbon in the soil is significant coherent, the coefficient, ρ (rho) is 0,73091. The average Boron content in all the samples is 0,53mg/kg.

Boron

n

Carbon (T/ha)

Low - < 0,5mg/kg

25

17,72

high - > 0,5mg/kg

37

21,72

Discussion

Professor Carlos Aguiar from the University of Braganza, considers the soil at Faia Brava as one of the poorest he has ever seen. Main cause is the cereal production during several centuries leading to a net export of nutrients from in the inland to the coast. When the organic matter content is very low, the water absorbing capacity of the soil is also very low. This means that when there is no rain in the dry, hot summer, vegetation has quickly a hard time due to shortage of water in the soil. Surprisingly the PH in the sandy soil was not so low. The soil analyzes showed that elements like Magnesium and Boron are often lacking and this will hinder the restoration of the ecosystem. The lack of sporadic elements like Boron will not be restored automatically…. and so restoration of the ecosystem is a tough process which will take a very long time. There is a significant correlation between Boron and Carbon in the soil. Thus with regard to the short term objectives, such as reforestation and production of fodder for the summer, Boron and possibly Magnesium will not recover in time, some supply may be needed as one of the interventions.

Conclusion

The present data have given us good insight into the soil quality of Faia Brava in 2021. We conclude that in addition to the natural processes extra intervention may be needed when ecosystem development on medium term (in 20 years) is desired. Therefore, soil analyses will be part of future reforestation projects and for long term monitoring purposes to follow the development of soil quality in time.

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