Impact of Plant Litters on Water-Holding Capacity of Slate Waste

1. Changes in water-holding capacity of fine slatewaste during decompositionof added plant litters. Mark Nason, Farrar JF, Healey JH, Jones DL, Williamson JC, Rowe EC. University of Wales, Bangor m.a.nason@bangor.ac.uk

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

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

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

5. Hypotheses 1) Addition of plant litter to slate sand will increase its water holding capacity 2) Leaf litter decomposition rate can be predicted from litter characteristics (C:N, %lignin) 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 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.

6. Experimental Design 4 treatments (litter amendments) Amendment Rate (dry g per 500g slate sand pot) None Alder litter 10 Birch litter 10 Alder and birch litter 5 + 5 3 replicates x 6 retrieval times (0, 2, 4, 8, 16, 32, 64 weeks) Completely randomised • 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

7. Collecting litter • Litter-trap under Betula pendula at Penrhyn Quarry

8. Methods 1: Measuring CENTURY equivalent pools LITTER >2000 mm Air dry soil Fresh soil <2000 mm 250-2000 mm PASSIVE Soluble C Microbial C 150-250 mm SLOW <150 mm ACTIVE

9. Methods 2: Measuring soil water holding capacity • Field capacity moisture 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 Available water content = moisture content at field capacity - moisture content at permanent wilting point

10. Initial litter characteristics 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 = 75.9 50:50 mix Alder:Birch %C = 50.9 %N = 1.7 C:N = 29.9

11. Alder and birch Alder Birch Control

12. Birch Alder and birch Alder Control

13. Alder and birch Alder Birch Control

14. 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

15. 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

16. Birch Alder and birch Alder Control

17. Birch Alder and birch Alder Control

21. Conclusions 1) All litter amendments have raised soil water holding capacity 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 5) Rapid transfer of C from CENTURY Litter to CENTURY Passive pool… …but can we be sure it is passive? 6) Decomposition of high C:N litter stimulated by proximity to litter with low C:N

22. Decomposition of high C:N litter (birch) is positively stimulated by mixing with litter of low C:N (alder) >2mm litter fraction Litter Initial C (g) 16 weeks C (g) C lost (g) C lost (%) Alder 4.88 0.57 4.31 88 Birch 5.39 2.69 2.62 49 Alder and birch 5.01 1.15 3.93 77 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.

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

24. What does this mean for restoration practitioners? When planting trees, think about litter! 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