Changes in water-holding capacity of fine slate waste during decomposition of added plant litters. Mark Nason, Farrar JF, Healey JH, Jones DL, Williamson JC, Rowe EC. University of Wales, Bangor

Background In natural systems, soil organic matter accumulation depends on the input of carbon as plant litter Soil water holding capacity is determined by Soil Organic Matter (SOM) content Establishment of plants in old hard-rock quarries is likely to be limited by availability of water Chemical characteristics of litters from different plants vary Rate of soil organic matter accumulation is dependent on litter decomposition rate Litter decomposition rate is dependent on litter chemical composition

Feedback mechanisms between plant growth and soil development CENTURY Soil C Soil Water Rain Plant C Photosynthesis Litterfall Drainage

Litter Metabolic litterStructural litter Active SOM Slow SOM Passive SOM CENTURY SOM Sub-model (Parton et. al., 1987)

1) Addition of plant litter to slate sand will increase its water holding capacity 3) The increase in soil water holding capacity caused by addition of low C:N leaf litter will be of shorter duration than the increase in water holding capacity caused by addition of high C:N leaf litter 2) Leaf litter decomposition rate can be predicted from litter characteristics (C:N, %lignin) 4) The amount of C in one CENTURY soil organic matter pool will be more closely correlated with soil water holding capacity than total soil C. Hypotheses

Experimental Design Senesced leaf litter collected in litter-traps during autumn 2000 Chopped to pass an 8mm sieve Thoroughly mixed with slate sand 5g woodland soil inoculum added Incubated outside 4 treatments (litter amendments) AmendmentRate (dry g per 500g slate sand pot) None Alder litter10 Birch litter10 Alder and birch litter replicates x 6 retrieval times (0, 2, 4, 8, 16, 32, 64 weeks) Completely randomised

Collecting litter Litter-trap under Betula pendula at Penrhyn Quarry

Methods 1: Measuring CENTURY equivalent pools Air dry soil >2000  m LITTER <2000  m  m  m <150  m SLOW PASSIVE Fresh soil Soluble CMicrobial C ACTIVE

Methods 2: Measuring soil water holding capacity Field capacity moisture content Available water content Soil soaked for 24 hours, covered and allowed to drain for 48 hours. Moisture content of saturated soil = field capacity Moisture content at permanent wilting point Water potential determined by Dewpoint psychrometer Moisture content when water potential is -1.5Mpa = moisture content at permanent wilting point Available water content = moisture content at field capacity - moisture content at permanent wilting point

Alder Alnus glutinosa %C = 48.8 %N = 2.7 C:N = 18.1 Birch Betula pubescens/pendula %C = 53.1 %N = 0.7 C:N = :50 mix Alder:Birch %C = 50.9 %N = 1.7 C:N = 29.9 Initial litter characteristics

Control Alder Birch Alder and birch

Birch Alder and birch Alder Control

Alder Alder and birch Birch Control

A AB B N A AB B N A AB B N A = Alder AB = Alder and birch B = Birch N = None Time 0 2 weeks 16 weeks

A AB B N A AB B N A AB B N A = Alder AB = Alder and birch B = Birch N = None Time 0 2 weeks 16 weeks

Birch Alder and birch Control Alder

Birch Control Alder Alder and birch

1) All litter amendments have raised soil water holding capacity Conclusions 5) Rapid transfer of C from CENTURY Litter to CENTURY Passive pool… …but can we be sure it is passive? 2) Duration of soil water holding capacity increase is positively correlated with litter C:N (in the short term). Litter C:N indicates litter decomposition rate and the litter fraction holds the most water 3) Initial litter characteristics indicate decomposition rate 4) Soil water holding capacity is more tightly correlated with CENTURY Litter C than CENTURY Slow, Passive or Active C 6) Decomposition of high C:N litter stimulated by proximity to litter with low C:N

>2mm litter fraction LitterInitial C (g) 16 weeks C (g) C lost (g) C lost (%) Alder Birch Alder and birch Expected C lost from alder and birch litter =69% Observed C lost from alder and birch litter =77% = 8% higher than expected Mechanism: decomposers use extra N (or labile C) of alder litter to decompose birch litter. Decomposition of high C:N litter (birch) is positively stimulated by mixing with litter of low C:N (alder)

Measuring soil formation at Penrhyn Quarry above ground biomass stem basal area litter input decomposition litter blow-out

What does this mean for restoration practitioners? Soil water holding capacity and rates of soil organic matter accumulation are determined by chemical characteristics of plant litter Slowly decomposing litter of high C:N will provide water to surrounding plants for longer Shrubs and understorey plants help retain litter on exposed sites When planting trees, think about litter!