declare @yourTable table (date datetime, startVol int, endVol int, diff int)
insert @yourTable
select '08/01/2012', 100, 15, 85 union all
select '08/02/2012', 125, 27, 98 union all
select '08/03/2012', 107, 42, 65 

;with cte 
as
(
       select date
              ,startVol
              ,endVol
              ,diff
              ,row_number() over (order by date) as rn
       from @yourTable
)
select a.*, isNull(a.startVol - b.endVol,0) as [import(New Field)]
from   cte a
left outer join cte b on b.rn = a.rn-1

OUTPUT:
date                    startVol    endVol      diff        rn                   import(New Field)
----------------------- ----------- ----------- ----------- -------------------- -----------------
2012-08-01 00:00:00.000 100         15          85          1                    0
2012-08-02 00:00:00.000 125         27          98          2                    110
2012-08-03 00:00:00.000 107         42          65          3                    80