I have been following the grassland rewilding discussions and scientific developments.
Grassland restoration is based on rotational, non-intensive mowing/grazing that is organized once or twice per growing season – usually, in autumn or even early winter.
Sometimes plant biomass is also removed in spring in order to activate seed germination and to promote seedling establishment (during the initial year(s) or restoration).
It is considered vital to remove any excess biomass, that is to say, to remove the cuttings from the grassland because grassland biodiversity depends on low nutrient levels and especially, low nitrogen levels that give a competitive advantage to some tall grasses and nitrophilous forbs (e.g., stinging nettles, goutweed etc.).
This is why the plants mown at the end of the growing season (as well as at any other time) are transported off the site to reduce the nitrogen as well as phosphorus concentrations on the grassland.
Of course, in rotational systems that promote structural diversity, not all biomass is removed because some patches are left standing and provide, e.g., dead stem shelter for certain overwintering invertebrate species.
If rotational grazing/mowing is applied, structural diversity is ensured which is why I will not be discussing the consequences of aboveground plant biomass removal from the perspective of life forms that need dead standing forbs and grasses.
Rotational systems is one of the answers to the problem that I have posed because they do not imply a complete removal of aboveground biomass.
(Meanwhile, in grazing systems at least some of the nutrients may be returned to the soil through defecation and urination by the grazing animals.)
However, I believe it would be important to pay closer attention to micro-nutrient circulation as well as organic carbon accumulation in grassland soils that might become limited if the aboveground biomass is removed from the site.
I am not entirely certain how micro-nutrients (such as Ca, Mg, Zn) are stored and returned to the soil level where they can be absorbed by plants.
For example, during the typical growing season (of perennial plants), the plant would invest energy and nutrients into producing stems and leaves, then – flowers and seeds and then the energy and the nutrients would be allocated to the root system to ensure overwintering and spring regrowth.
Most references that I have found discuss various types of carbohydrates that are transported and stored in the belowground parts (roots, rhizomes, adventive buds etc.).
I have not yet come across information where micro-nutrients are stored during each of these stages of resource investment.
Sometimes micro-nutrients – when reaching abnormal concentrations – are regarded as heavy metals and plants that absorb and contain them are involved in the bioaccumulation processes.
Most of such references discuss storage in aboveground organs although some have referred to roots and rhizomes, as well.
If calcium, magnesium etc. are stored in the aboveground biomass rather than in the root system, removal of the aboveground biomass would result in a constant and gradual decrease in the local micro-nutrient availability on the particular site.
Nutrients are cycled through the absorption by roots, transportation to plant organs, containment within the dead biomass, degradation and return to the soil.
If the dead biomass is not returned to the soil, locally, the micro-nutrient cycling might become affected in an undesirable manner.
Not only are micro-nutrients important for plants, they are also sought for by herbivores (and herbivore predators).
For example, if the plants locally do not provide sufficient micro-nutrient values, the herbivores might suffer fitness consequences which could be reflected in their reproductive rates and population sustainability.
Also, if herbivores cannot obtain their nutrients on grasslands, some species that are mobile (e.g., ungulates) will seek the resources elsewhere and this could cause damage to agricultural crops, orchards or forestry (for example, nutritional deficiencies can result in increased bark stripping by large herbivores).
Thus, the reverberations of micro-nutrient loss can be of wide scope.
Another issue concerns the organic carbon content in the soil.
Nutrients that are not beneficial in grasslands (in excessive amounts) are nitrogen and phosphorus.
However, micro-nutrients and carbon might have to be separated from these ‘unwanted two’.
Carbon (and especially organic carbon) is essential in many vital soil process such as water retention, erosion control (through substrate formation), nutrient cycling, soil flora and fauna associations (soil fungi are crucial for plant development and some plants that are rare and endangered depend on soil fungi that increase their competitiveness and seed production rates) etc.
Carbon concentrations would, too, decrease with removed aboveground dead biomass and the results might not be desirable because soils without organic carbon are not as functional as soils with organic carbon (see, e.g., Trivedi, P. et al., 2018).
Excessive fertility is not promotive of grassland biodiversity.
However, organic carbon ensures a certain functional fertility where the causes for successful plant growth are not… rooted in nitrogen and phosphorus (fertilizers as we understand the term) but rather in soil organic carbon that establishes the mechanisms necessary for highly efficient water and micro-nutrient cycling as well as floral and fungal associations at the root/rhizome level.
How to resolve this dilemma of the necessity to remove nitrogen and phosphorus and to ensure that the soils are functional, productive and not limited in micro-nutrients and essential soil organisms?
The solution that I see as viable is removing the aboveground biomass and composting it nearby (so that the composting system consists of the plants gathered in the area but is also exposed the local soil organisms that participate in the decomposition and that are later returned to the soil).
After the composting process in which nitrogen concentrations are decreased, the compost might be used as an organic fertilizer (in the form of humus or as ‘compost tea’ or as ‘distilled’ supercompost that rather aims to introduce important fungi rather than to fertilize the soil).
This could provide benefits of retaining and promoting importan soil microbiota associations.
If some plant species are in greater need of fungal helpers, these plants could be removed from the grassland selectively with the associated soil organisms and they could be planted in the compost so that their roots spread and so did the fungi.
Later the individuals could be used as plug plants and the compost would also have their root symbiotants (although the compost would be depleted of some nutrients that would have been uptaken by the plants).
The plants could also be grown nearby and not on the compost itself.
Upon replanting, some of the soil from their roots could be introduced in the compost where the microorganisms could spread and whence they could be distributed on the grassland.
Or microorganisms could be introduced simply from sampling soil near the respective plants on the grassland and inoculated in the compost in hopes that they would participate in decomposition of the biomass and multiply (this, however, might not be a workable method for organisms that form associations with living plant roots and that are not merely decay agents).
Composting the removed biomass and returning the compost to the soil might solve some of the micro-nutrient and organic carbon cycling issues in grassland restoration schemes.
Notes by Ieva 