Nutrient additions in forests

3.2. Nutrient additions in forests

In forestry, nutrients are sometimes added via fertilisers, lime and wood ash to the soil in order to increase the wood production and to compensate and return nutrients taken out when harvesting.

Fertilisation

Fertilisation will normally lead to an increase in the soil nutrient stock and in this way improve the capacity of the soil to produce. The aims of fertilisation are: i) to gain a short-term positive growth response especially ameliorating harvesting-related reductions in growth and ii) to secure SQ in the long-term especially securing the nutrient balance after organic matter removal through intensive harvesting.

Stem-growth increases as a result of fertilisation with N, P and K has been observed [197, 193]. However, [197] furthermore observed that fertiliser additions caused a significant decrease in foliar concentrations of all nutrients except for N. After fertilisation, [15] observed a short-term rise in soil pH, which later turned to a long-term drop in soil pH. Both fertilisation and the presence of N-fixers caused a marked increase in the concentration of soil N in the A horizon [93]. However, [140] and [161] observed no consistent effect of fertilisation on the soil nutrient availability in White spruce and pine, except for Ca. If N is added alone or in too high amounts, a negative impact on the balance of the other elements, mainly P and the base cations, might emerge, either due to a positive growth response or due to increased leaching [80]. This effect may be relevant e.g when using biosolids derived from municipal sewage sludge or from mill residues as organic matter amendments, especially to less fertile soils [201, Johnston and Crossley 2002]. The content of N and P in these amendments is normally rather high, which in relation to N saturation and following base cation leaching may have serious negative effects on e.g. the soil Ca and Mg stock. The use of sewage sludge and mill wastes is furthermore controversial because of potential contaminants such as trace metallic or organic elements and will continue to be until all risks for accumulation in the ecosystem, transport to adjacent waters and transfer to humans have been eliminated.

Liming

Positive growth effects of liming in agriculture have for more than 100 years inspired to look for the same gain in forestry. Many forest experiments have thus been performed but often no positive growth response has been observed. To our knowledge a thorough international review has not been done.

However, a comprehensive review of Finnish liming experiments in Norway spruce and Scots pine showed negative growth response of liming when not followed by other fertilisation [51]. In Sweden, growth effects of liming included both positive and negative short-term response [165]. An immobilisation of N in high C/N-ratio raw humus was suggested to explain the negative effects.

Accelerated leaching of Ca and Mg following deposition of acidifying N and S compounds reinitiated many liming experiments in the 1980s and 1990s. Especially, Mg shortage initiated by acid deposition seems to be compensated by dolomitic liming [112].

In principle, liming should improve soil pH, the Ca storage, and the Mg storage if dolomitic lime is used [112]. Absence of positive growth response could be caused by other growth factors like N availability or drought or by negative effects of liming neutralising or hiding a possible effect. Such negative effects might be leaching of N, negative microbial effects etc.

Lundström U.S., Bain D.C., Taylor A.F.S. and van Hees P.A.W. (2003) Effects of acidification and its mitigation with lime and wood ash on forest soil processes: a review. Water, Air, and Soil Pollution: Focus 3, 5-28.

Löfgren S., Zetterberg T., Larsson P-E, Cory N., Klarqvist M., Kronnäs V., Lång L-O, 2008. Skogsmarkskalkningens effekter på kemin i mark, grundvatten och ytvatten i SKOKAL-områdena 16 år efter behandling. Skogsstyrelsens Rapport nr 16, 123 s.

Wood ash recycling

With current practices, the increased use of forest fuels results in an intensified export of nutrients from the forest. A large part of the forest fuel consists of branches, tops and needles that were earlier left to decay in the forest. Although these fractions only amount to a small proportion of the total weight of the tree, they have a much higher nutrient concentration than stemwood [164]. Thus, the increase in nutrient export might be significant. Another undesired effect of the nutrient export is enhanced soil acidity. Returning of wood ash after incineration of wood has therefore become relevant. The principle aims of recycling of wood ash to the forest are to i) avoid depletion of essential soil nutrients and to ii) reduce the harmful effects of acidification of forest soils and adjacent waters [9].

The major components of wood ash are Ca, K, Mg, silicon (Si), Al, iron (Fe) and P as well as trace elements, some of which are toxic [149, 199, 86, 60, 107, 24]. Ash is generally low in N and S because it is vaporised during combustion. Due to different soil mobility of toxic elements like cadmium (Cd) and caesium Cs), caution must be taken when wood ash is applied to forests. The ( chemistry of the wood ash is dependent on the tree species. In general, ash from deciduous tree species contains more K and P and higher proportions of macronutrients but less Ca and Si than ash from coniferous tree species and is therefore likely to be a more effective fertiliser [223, 164].

When wood ash is applied to a soil it will raise the pH of the upper soil. Untreated ash gives the largest and most rapid pH increases and the higher the dose the higher the increase in pH. The effects of wood ash on the acidity of soils seem to last over a long period of time. Ash doses around 3-5 t ha -1 have

been shown to elevate pH 1 to 2 pH units in the XX layer 10-19 years after application [143, 138, 30, 181]. The transport of ash components down through the profile is however slow and the effects deeper in the profile are found to be small and usually only occurring a considerable time (>10 yrs) after the application of the ash [30, 181]. Hence, an increase in the pH of mineral soils is not usually

found [177, 10, 66] except when high doses (>10 t ha -1 ) have been applied [97].

The content of both K and P in wood ash seemed to be lower than from commercial fertilisers (K: 65- 70%; P: 28-70%) [148, 164]. Some elements in ash are quickly leached with the percolating soil solution. Elevated concentrations of K can be found in the soil solution at deeper levels shortly after The content of both K and P in wood ash seemed to be lower than from commercial fertilisers (K: 65- 70%; P: 28-70%) [148, 164]. Some elements in ash are quickly leached with the percolating soil solution. Elevated concentrations of K can be found in the soil solution at deeper levels shortly after

8 t ha -1 wood ash was applied to a Norway spruce forest, the effect on soil and fine roots were followed [35]. An increase in soil exchangeable Ca and Mg and these elements in fine roots along with

a decrease in Fe, Zn and Al in the soil exchangeable fraction were observed. Furthermore, pH increased from 3.2 to 4.8, base saturation increased from 30% to 86% and BC/Al ratio increased from

1.5 to 5.5.

As long as N remains the growth limiting nutrient [204], the addition of other nutrients through wood ash will not increase growth on mineral soils. On the other hand, wood ash addition in forest stands on nutrient rich peat soils has shown a significant positive effect on tree growth [63] and improved conditions for natural stand regeneration [88, 122, 123]. Peat soils deficient in K and P but with a good N status show the highest increase in tree growth [191] while tree growth on peat soils low in N (<1%)

3 remains low [188, 189]. An increase in productivity of 3-4 m -1 ha over 55 years was found after wood ash applications of 5 t ha -1 to drained peatland [190, 109].

A future task is to decide for an optimum wood ash application. [6] recommended an application of wood ash to achieve a humus layer base saturation of 50% and a mineral soil base saturation of 20%. It is necessary to apply different doses to different tree species since tree species such as beech, oak, Norway spruce, Scots pine and Douglas-fir typically seem to grow best with base saturation rates of 30% while hornbeam and sycamore are likely to prefer levels in excess of 50% [164]. A granulated form of ash has been tested since it is easily spread and creates a slow release of chemical elements and thus reduces the risk of alkaline flushes through the forest soil [164]. In Sweden, granules are not considered efficient in a larger perspective when also including the stabilization process of ash and its economy.