Effects of gravel mulch on surface energy balance and soil thermal regime in an unheated plastic greenhouse

Abstract

This work analyses how a mulch layer of mostly gravel particles affected the soil thermal dynamics in an unheated plastic greenhouse in a Mediterranean area (South-East Spain), where suboptimal regimes of soil temperature usually prevail in winter crop cycles. Soil temperature and heat flux profiles were measured in two soil types: (i) a 0.1-m thick gravel mulch (GM) placed at the top of a 0.30-m layer of imported loamy soil and (ii) the latter without gravel mulch (NM). These measurements were conducted during a winter period (14–28 January) when the dominant source of energy in the soil root-zone was the heat from deeper soil layers. The higher albedo of the GM and its higher long-wave radiation losses reduced substantially the daily net radiation at the mulch surface by ca. 77% with respect to NM. However, the soil root-zone maintained warmer under the GM than with NM in a period when soil temperatures are usually below the optimum. This was mainly caused by the insulating property of the mulch, which acted as heat barrier and increased the resistance to heat transmission from deep soil horizons towards the surface. This passive heating of the soil root-zone by using a gravel mulch was later confirmed in a summer–winter cycle of sweet pepper grown in an unheated plastic greenhouse. The gravel mulch also reduced the soil evaporation and increased the reflected short-wave radiation towards the plants, but the air temperature above the mulch fluctuated more strongly than in the absence of the mulch.